A lot of great observations in this thread.
I have two very close INFJ friends. I also have a number of good INFP friends as comparison points. I'm not quite sure why there is confusion between the two types other than basic introversion; to me the Fe/Fi differences pop out quite quickly, unless
the INFJ doesn't have as much Fe development and then the Ni can appear Fi-ish.
What I notice is that INFP seems more "flexy open." They are more obviously tender-hearted in a "raw" sense (I don't know how to explain it better). Any walls I run across in exploring/interacting with them are "fuzzy walls," if that makes sense, I can poke and prod a bit and sense some give there and get a feel for what might be in it because of how the wall flexes around that hidden knowledge.
With my INFJ friends, there's just a veneer of control and a cordial wall / way of doing things around them. They are very kind but read "cooler" to me than the INFP counterpart. I also feel like certain areas of knowledge are very blocked off while others are very open. The walls are much more firm and I only get access to a hidden area if they come right out and tell me what's there.
I'm more careful with most of my INFJ friends because I don't either want to run into a wall and hurt myself or else go completely into an undesired location and have them shut me out. With my INFP friends, I feel I have more leeway -- they have a playful, fuzzy way of flexing around me if I intrude too far by accident, and they might give a bit or throw me something to see what I'll do with it. INFJ seems far less prone to do that, even if I put myself out there first; the decision has usually been made up front about what information I will be allowed access to. INFJ feels more engimatic to me, I usually can grasp what lies underneath in my INFP friends more easily.
Interesting that you say this because yes I do see the INFJ like that but perhaps because of some sense of flaming impunity I have, those boundaries aren't "real" to me, more like markers of progression. I think this is maybe some type of Fe-Fe interaction and I'm not sure how to explain this, but it's the Fe way for relationships to follow some sort of linear path and to categorize the state of the relationship. Is this a date? Are we dating? OK, we're not acquaintances anymore, I can call you a friend and later on if things go well, you'll be a close friend, confidant, queue theme from Golden Girls. You're my coworker and I don't want to go there with you so I'll maintain a friendly distance.
I'm not sure if I'm doing a good job explaining this, but depending on the where and how the relationship is filed it requires a different sort of interaction. That filing may be interpreted as a boundary or at that particular stage of the relationship Fe uses the boundary as stopgate. That stopgate basically is either open or closed, it's not ever just ajar. If it's open, it'll stay an open border (Fe not able to detach). When it's closed, well it's just closed, no problem with attachment or detachment because it never was there to begin with. I think many people complain of this type of Fe being rather empty and superficial, but once again, it's not empty to me and when I get it back from other FJs it's not offensive or weird to me.
Personally, I appreciate those clearly marked areas. I look for them, I see them, I usually respect them (unless I don't, lol), and I expect them to be present. I think when you get beyond the superficials and all systems seem to be go but you're still feeling something is there, the boundary isn't real. I'm gonna have to break into the SAT words but to me, I perceive that boundary as a simulacrum. It's like when you're on a movie set and it seems like it's a brick wall in front of you but if you apply any sort of pressure to that brick wall you find it's really painted styrofoam. If this is Fe, it's basically Fe looking for proof that investment will be reciprocated. I don't want to speak for INFJs, but my Fe is very reciprocal and you need to be thoroughly investigated, researched, and vetted by reputable sources and until then it's very measured and tit for tat. That may sound cold to people, but I view it as a self-protective measure. I don't just see what happens or just try it out because something may develop.
I'm not going to feed into the Rare Bird INFJ that needs special handling. Getting close to an INFJ is not some big mystery. If you encounter a person who happens to be of the INFJ type and if they're making you climb mountains and swim through ocean trenches to get their hallowed friendship, move the hell on. With my INFJ friends, I didn't work extra hard to do anything. I think we clicked and I didn't find them to be particularly closed. We moved through that Fe path actually I see them being as brazen as possible to scare people off using that brazenness as a way to distinguish who will be a true friend and who won't.
OK, so about that stopgate. With the INFPs I know, (more NFPs than SFPs), I don't necessarily get the feeling of the stopgates and I know about the whole Fi value trouncing=saber tooth tiger and all (I'm not even quite sure what that means anymore but whatever) but the signs that I would typically look for as indicators of a deepening relationship aren't really consistent. After awhile of knowing the particular INFP you figure out what's what, but up till that point it's groping in the dark and often strikes me as being unfair and unnecessary, but I can see how to the INFP the signs I would understand would probably be interpreted as contrived.
I wrote this about a (former) INFP friend of mine a few months ago in this thread:
http://www.typologycentral.com/forums/nf-idyllic/18803-fi-selfishness-related.html Though I think my friend was behaving like an unhealthy INFP it might shed light on some INFP/INFJ head-butting issues.
I remember that this thread started a whole discussion about how I am being too demanding of my friend by expecting her to behave in certain ways (my Fe I suppose) while I am thinking that one can expect basic things from friends else they are nothing more than acquaintances and in order to deepen a relationship the two parties have to invest in each other, giving of oneself, one's time etc else they are always going to be swimming in the shallow end of friendship.
Yes, I remember that and don't think you were being unreasonable at all.

From my personal experience, I think it's important for an FJ to have another very close FJ friend (I find that ExFJ/IxFJ pairs work better than ExFJ/ExFJ not sure about IxFJ/IxFJ) because IME another FJ won't find the level of investing in each other to be extravagant, needy, clingy, or dependent and it's expressed in a way that that two FJs in question can understand and appreciate.
I started a thread awhile ago about FJ/FJ relationships and I'll just throw this in this thread since I've got a bunch of FJs here. I know this is going to seem very teenage-girlish but I think it's common for two FJs to click up very intensely. People call my INFJ friend and I Double and Trouble. We're inseparable even outside of work. I find that there are no boundaries between us, even when you want the safe haven of a boundary to hide beneath. And it's not even like we just sit around confirming each other's world view, there's honest feedback, i.e. "I don't think what you're doing is a good idea", "I don't think you're handling it in the right way," "I wouldn't have done that" and it
does not come across as preachy or overbearing. I interpret this behavior as something a true friend does. To me, it feels like I'm never really far from her mind or thoughts so if I get a text asking me if I remembered my doctor's appointment or if I ask her did she update her resume yet to reflect a recent conversation we had, it doesn't feel claustrophobic, it feels caring.
When either of us is laying it on too thick with the other, we have little signals that lets the other know back off and we move on without a second thought given to the exchange. I enjoy the very high level EQ we have towards each other. I find it to be a very open, honest, sensitive, caring, and supportive friendship with minimal conflict and misunderstanding. I think that if I behaved the way I do with my INFJ friend and she with me towards other people, we'd quickly get labeled bossy but that is not the case between us. Neither one of us feels the need to put ourselves on mute and we like to talk about our friendship, it's like we meta-analyze our friendship to keep it running smoothly and we enjoy doing this; it doesn't feel like work. We know it looks strange to other people but the feeling of being able to say anything and be yourself completely with another (ugliness, bitchiness, and talons included) is so refreshing that I'm totally willing to talk the hell out of it to keep it going.
Unfortunately, I can't go into this level of detail about my INFP friends because we've never gone there with each other. The friendships don't seem to be on negotiated terms; there's no mutuality. It's either on or off and it's basically up to the INFP when it's going to be on. When they're there, everything is cool, we get along great, excellent EQ and sensitivity as well but otherwise: ????
Which is why this makes so much sense to me:
INFJ: I really need you.
INFP: I'm really really down today as well. Bad day bad day bad day. I'm sorry but I can't right now. I need to be alone.
INFJ: Well you know what? If you were in my position.. I'd be there in a heartbeat.
INFP: I know, I'm sorry. I just can't. Not today.
INFJ: Just don't count on me helping you out when you're the one who needs me.
INFP: ...