While I think some things are issues of over-sensitivity or immaturity, there are some ways we see the world that are sometimes misunderstood by those we try to interact with. Knowing those things would smooth out some of the initial problems that might be encountered. From my perspective, these are some of the things that might be useful to know. I realize that not all INFJs are the same, so these are gross generalizations.
1. You may be surprised as you get closer to an INFJ that they seem to "become" much more opinionated than you had originally thought. They may also seem less easy-going than you had originally believed them to be and they bring up differences between themselves and you. This is actually a compliment. These are signs that 1) they are invested enough that you are worth being honest to or having conflict over the things that matter 2) they trust you with ideas they hold dear or feelings that could be trampled.
2. INFJs often have the need to bring up conflict from a slightly different angle after you already thought it was done with. They are not trying to win, or nurse hurt/anger or make you feel bad. It's like cleaning out a drawer and finding a few items later that they don't know where to put and they need your help knowing where they belong. Or, like discovering another splinter a couple days after they thought everything was removed. Even though it's a pain to get out the tweezers again, if you don't it will heal over, then get full of puss and fester. They are trying to avoid the emotional equivalent of this and need you to help them. (Obviously, they need to prioritize which things matter enough to do this with as it can be quite wearing for the other party. However, this often seems to be an area of misunderstanding and if the other people at least understand the motivation, it seems to help).
3. Mature INFJs are quite emotionally strong. However, sometimes they have a tendancy to suck up more than they should or can because they want to be sure it is not just them being oversensitive or hasty in judging. They also really dislike crying in front of people. In times of tiredness or extreme emotional distress, the dam breaks and they may cry or get upset (usually without wanting or meaning to). People around them are a little bewildered at this seemingly "sudden" change in them. Under no circumstances pick that time to tell them that they are overreacting, are oversensitive, don't make sense, etc. And don't make this all about how their tears make you feel. Give them a hug, listen sympathetically (realizing you are doing a great deal of good just be doing that) and revisit it later.
Sometimes they have avoided bringing seemingly little incidents up because they don't want to seem oversensitive or make the other person feel bad. After quite some time though, they may notice that all of these incidents had the same underlying theme and that is why they were a problem. In an unguarded moment, one little incident can touch off the whole pile of firewood! They sometimes need to express these frustrations or fears sooner before there is a lot of resentment or hurt built up.
4. Venting or talking is their way of processing what has happened. They'll sometimes forget to tell you that the best thing you can do is just listen without offering advice (yet). They will shift towards finding a solution on their own (and will ask your opinion) when emotional excess gets cleared away or when they've had a chance to clarify their thoughts. It's like a tree that has fallen across the road. They need help getting it cleared away before they can go anywhere. If you seem to take someone else's point of view or try to offer a solution without understanding the problem first, it is like not only not helping clear the original tree of the road, but felling more of them. You are actually delaying the process you are hoping to speed up!
5. Expressing personalized appreciation for an INFJ goes a long way with them in every context, as long as it is sincere. They shrivel up without appreciation and become resentful or discouraged even if they don't express it.
6. Give them time to get used to an idea and they will be much more open to it. They have to check it in their internal idea structure to decide if and where it will fit in. This is true for food, activities, ideas and sometimes even people. However, INFJs do appreciate someone who introduces them to new things and will thank you for it later as long as you don't try to do it all at once.
7. INFJs tend to be open to being approached, but are less likely to do the approaching themselves. Assume they will be friendly and go from there.
8. Integrity is a huge value with INFJs. The closer you are to them, the more they will expect in that sense. It may seem then that they are more accepting of people with much worse behaviour. The difference is that they don't really care about those people, so what those people do matters far less than what the people in their inner circle do.
9. INFJs will give you much more space if you let them know what's going on, or verbalize some of your thoughts/thinking process. While INFJs are fairly independent, they really do value time with those close to them and want to know that their people feel that way as well about them. If they do not get that, they are likely to assume the worst, instead of that you are lost in your own head, are busy with a project etc. They will keep hanging around trying to make sure that home base is securely established before wanting to venture further away. If you need some space, just let them know what's going on and when you are likely to be more available.
10. Because INFJs listen well to others, others often forget to ask them about themselves. If you want to get close to an INFJ, remember to ask some questions back. INFJs are usually pretty open. Be careful not to debate for the fun of it or laugh at something they tell you or something that is important to them. INFJs usually do not share thoughts until they have been processed and assimilated into their thinking. Therefore, unlike some types that will throw out an idea and try different perspectives on for size, that rarely happens with an INFJ. Their ideas, activities, work and choice of friends are an outgrowth of who they are, so if you do this, it is perceived as you making fun of them personally. You won't get ushered into the next "room" of their inner world if you go in with muddy shoes to the room you were allowed to see! Probably the biggest compliment you can give an INFJ is wanting to understand them completely. They're pretty forgiving if you did mess up, as long as you acknowledge it and adjust your behaviour.
11) Sometimes INFJs tend to agree to something too quickly under pressure and think over it after and change their feelings about it. Or they will overcommit or unreasonably inconvenience themselves in an attempt to satisfy everyone. This in fact, frustrates many people and they may not be aware of it. They also sometimes have trouble drawing boundaries for others' behaviour as long as they understand it (they do know when they're being taken advantage of). I think this improves with maturity. I think they do it because they know what they can handle emotionally and would rather deal with something they can control, even if it inconveniences them, rather than leave the negative feeling for the other person, because they can't control that.
12) INFJs will get over a relationship much faster if they are given some closure. If you want to save yourself some headache, get the talking done before you leave!
13) INFJs can be easily embarrassed. This is part of their reluctance to try something until they can do it a little bit, or their need to watch before joining in. For some, it may make them less interested in team sports vs something like tennis or badminton. It also can apply to social situations. If you are teasing an INFJ, it is best to do it privately or make sure first that they feel quite safe with the people you are teasing them around. Otherwise they may act like they're okay with it and then quite unexpectedly crumple on you (which makes you feel like a heel and they really don't mean to!).
14) Just because they seem "nice" to a lot of people or they don't always vocally disagree, don't assume that you know how they feel about everything, that they don't observe and have unexpressed opinions about people, that they are pushovers or that they agree with yours or others' opinions. It usually has more to do with not creating unnecessary conflict (people aren't close enough to matter) or else gathering enough information and being sure of their perceptions before saying something. Occasionally it may mean that they've repeatedly tried bringing something up, with bad results ensuing and are feeling discouraged and hopeless (in that case, they'll just wall off that part of their life to you).
Conflict usually comes up 1) if this is going to be an ongoing problem (at work, with roommates, or a significant other) that needs to be cleared up. 2) if you are close enough for them to care about what kind of person you are or be distressed by an unresolved problem between the two of you. 3) under extreme emotional (and sometimes physical) stress. Usually it is an existing problem that they thought they could just handle on their own, but are temporarily lacking the resources to do so effectively.