INTJ issues as observed by an outsider:
1) The Death Stare
INTJs have this non-descript look on their face which makes most people rather...unnerved around them. Are they approving of you, disapproving, is there something on your face, what?? Most of the time however, they are just taking in the world in their own detached way, collecting data and drawing conclusions. And most, if pressured to answer what they were thinking at the time are unable to voice it, as a) their si doesnt let them recall it and b) the information hasn't been synthesised yet. Worse even is asking if they liked what they saw, as it doesn't occur to them to run it by their Fi unless specifically prompted to do so, or unless it was something they *really* disliked. Which brings me to point two:
2) Extreme neutrality
There is nothing worse than getting an INTJ to relate to you how he feels about something. It is like trying to get blood from a stone. He'll be able to tell you within a flash what the cost and benefit analysis of any given situation is, and how it will likely turn out but do *NOT* ask him how he feels about something. Unless something causes a visceral reaction, you are likely to get 'meh' as an answer, if they bother to answer at all. Like, dislike, and shrugging are all the same thing to them. Rarely will they admit they love something, and when they really hate something, you'll know within a flash. This often gives the impression that pleasing them is impossible and he seems to be out to get you when it comes to voicing what is missing or what he doesn't like

The only exception is when you are privy to their utmost inner feelings and even there, it can be a lot of work to get them to drop their guard to the point of childlike glee. That glee however is well worth the wait
3) Allergy to repetition
Since INTJs are so good at synthesising and assimilating information (despite their lousy memory) they *hate* repetition as it is a highly inefficient format to them. So please try to refrain from telling him stories which are familiar, which he has extracted every ounce of information out of already and which have sentimental value to him as he will completely be oblivious to the fact that it is the familiarity he is supposed to enjoy as well as the emotional information attached to said story, and not the facts which he has already assimilated before. Since they are so future-oriented, living in the past, or recalling past anecdotes feels foreign and confusing to them. Their indulging of you doing so, will highly depend on how much they care for you, and seems to always have a taxing and grating effect on their patience, however substantial their patience can be. Make sure you appreciate the fact that they do put up with this as it is a sign of politeness and often appreciation of the 'other stuff' you have to offer. In short, it shows that they appreciate you as a person far more than you are likely to suspect.
4) Their lousy memory
Forgive them for their inability to remember your romantic moments together. Or any moments together, for that matter. Since their Ni synthesises every bit of information without retaining the details, they will often only remember the important result that came from those moments together. Recounting the stories is one way of getting them to remember, but then they are likely to that it is something that apparently is important to you and will recall what you recount (as it sets of their repetition radar), so they can stop you from having to do that again

This however, ime, does not mean that they do not care about the time you spent together, but they seem to subconsciously value said time and project what they experienced with you into the now and into the future together you will build.
5) Their cost benefit analysis as the cause of their patience
They can be extremely patient for a reward, given that this reward is well worth the wait. Though they can be really short and impatient in situations that hold no real value to them, they can put up with the most gruesome circumstances, as long as the pay-off is well worth it to them. This makes them also extremely realistic when dealing with situations and even people. Unless Fi gets out of control, they are perfectly capable of seeing, for instance, their friends and their partner for who they are: imperfect people who are well worth the obnoxious stuff they bring along for the benefits they bring. The same goes for life in general. INTJs are nothing if not adaptable to reality, especially when it comes to obtaining that carrot they want in the end. It also means that their partner, however flawed s/he may be, will never be blamed for who they are in the relationship. The intj knows what he signed on for and who he started dating and will deal with the obstacles along the way in their usual confident way.
6) Their need for efficiency and the kick in the ass they need to appreciate silly things
As an introvert, they have limited resources, energy-wise and are well..lazy out of necessity, or so I'm told. Efficiency is the name of the game. Do something right, thorough and competently and you will only have to do it once. This seems to be their mantra. And this is also what they often specialise in. This causes them to sometimes forget that *others* do not work that way, nor see the priority in this. This often results in much irritation, criticism and frustration on their part as well as an inability to do something just..for fun. While they are certainly capable of this, ime they often need to be prodded and poked for fun in order to get them to realise that life doesnt always have to be this serious and that it is ok to just do things for the heck of it, to enjoy yourself. Once they learn this, they seem to gravitate to anyone who can provide them with that kick in the ass, towards those that can provide them with entertainment. Granted, their favourite part is still watching the drama from the sidelines with some popcorn, but one can motivate them to actually act silly themselves given enough encouragement. In time, they come to appreciate people who provide release for this side of them very much. Until they discover this part of themselves and come to terms with it however, criticism and efficiency will be definite stumbling blocks when interacting with others.
With this comes the fact that if an INTJ takes the time to initiate a interaction with you, there is a reason for it. Either you have information they want/need, you are essential to getting on of their missions to work, or....they actually like you as a person and value you for what you in general can contribute to their lives. The latter is quite rare and makes me smile every time *grin*
7) Their ability to see what is missing and their inability to keep quiet about it.
INTJs are brilliant at seeing what is missing. You clean the entire house, but they come home when you are still finishing up and the first thing out of their mouths is: 'Why are the scissors lying there?' Nevermind the fact that you just decluttered the entire living room, those scissors apparently are not supposed to be there!!! At that point you just want to throw a pan at their heads. Refrain from doing so, and count to 10. Realise that your INTJ left a cluttered house where clutter was the standard, and came home to an organised house where only the scissors apparently doesn't fit into that context. So he notices it. Grating as it may be that he does not even show appreciation for all your hard work, it is something they cannot help. They cannot help to notice that one disorganised thing. Often, when you yell at them, they seem to realise that appreciation for your hard work was the better first response and will try to make amends as they *DO* appreciate the time and effort it took you. It is just not their first priority

It helps to keep in mind that their ability to notice what is off in a situation is what makes them so talented at what they do in life, usually..and that it is one of the reason s you love them, especially when trouble comes knocking.
8) Their inability to feel out social situations
Most INTJs prefer to be alone and have been taught by life that if you want something done right...you have got to do it yourself. Self-reliant as they are, people and social interaction kind of takes a back seat. This also means that they are completely oblivious to what social protocol entails. For them, when they visit you at your parents house, it is perfectly acceptable that your parents do not approve of him, especially when he overstays his welcome. Pleasing others they have no interest in isn't really on their to-do list, even if it could be socially advantageous (they grow into this eventually though, but in a very calculated way *lol*). He is there to see you, and as long as you are happy to see him..the mission is complete (and he will only leave at your urging). This also means that concepts such as bringing soup to those that are sick (unless they indicate they need help or are an extension of themselves, aka his partner/kids), worrying about how others will respond to his actions (unless they will pose a logistical problem which will cost him more energy), doubting himself and his capabilities coz others put him down, etc will not bother him. And will bother some others greatly
9) Their tender heart
Underneath all that confidence, competence and ability to weather the most horrifying circumstances beats very squishy, vulnerable heart. They just make sure it is insulated really well against the outside world. Therefore, if you happen to push in the right password, and find yourself allowed into this sacred vault...please, please be careful with its contents as its fragility rivals its beauty. As tough as they are on the outside..thats how fragile they are on the inside. While an INTJ is perfectly capable of never opening that vault for anyone, it truly is a waste to never let those contents see the light of day. With the right person, the light they keep inside, locked away, afraid that the world will abuse it in some way, can come shining through in a way that is truly mesmerising. There is nothing quite like beholding the childlike innocence coupled with that kind of fearsome competence. The people who get to see the contents are often family and loved ones - people who become an extension of the INTJ himself. Should you be elected for this honour, you will find yourself better protected and cared for than you could ever imagine, as they take care of you the way they take care of themselves - something they take very seriously.
Occasionally, some things will bubble to the surface, giving you a rare glimpse of what is likely to be inside, and, if not treated with the care it needs, can cause quite an uncharacteristic moment of pain for the intj. While they are very good at withdrawing at that point and covering it up, the pain and vulnerability will be written on their face..if only briefly. Amends can be made, but the damage will take time to repair before you are allowed near the vault again. While they do not expect the outside world to be careful with their hearts, ime most still yearn to be understood and cared for. They've just come to terms with the fact that the outside world is unlikely to be able to give them that.
It goes without saying that destroying the contents of the vault is likely to cripple and damage the INTJ severely. Often, it confirms what they feared to be true: other people cannot be depended on or trusted. The only one you can trust is yourself. Undoing said damage is...a painstakingly slow and energy-consuming effort, both for the INTJ and the party attempting to bond with the INTJ, as it triggers fears and anxieties in traumatised SiFi.
10) Childlike glee
Once inside, you will get to witness the childlike glee that occasionally just...bubbles to the surface. Your INTJ will suddenly start bouncing off the walls, come up with all kinds of crazy activities he wants to do NOW, with a petulant impatience that is *so* unlike him, it catches you off guard every time. Suddenly you are dragged out to go drinking, where he acts like an ESTP to the waiter, you get woo-ed with roses and champagne, or get left behind for this new awesome cool toy he just *has* to have and figure out, right now. This obsessive behavior can last from anywhere of a couple of minutes to several days -until his work calls for him to return back to the stable competent man you have built a life with. With time, the befuddlement will diminish (but never fade fully) when he gets this mood, to the point where you cant help but just smile when this side of him appears.
....I think I'll stop here, for now