I also have some select ISWAK thoughts but I'll post 'em under the Asian fandom friends filter because I know nobody else cares, probably. :)
This, like I mentioned, isn't anything radically new or amazingly clever of me to figure out but anyway, I got to thinking how in fandoms, people tend to label themselves with the specific things they like and dislike about canon. In Harry Potter, people can be "Gryffindor fans" because their main interest is in the characters who're in that house, or Marauders fans or Death Eater fans or whichever. In Asian pop fandom you can identify yourself as a fan of some group in particular, or a type of Asian pop - like some people could only like Korean boybands under SM Entertainment label or something or only Cantonese pop or Japanese female vocalists or whatever. Or in Green Wing fandom, some people clearly are in the fandom mainly because they find Julian Rhind-Tutt mad hawttiez (and the show hilarious but their main interest is still Mac/JRT).
There's another side to this, of course, like the fact Gryffindor fans will not understand Slytherin fans. It's just the good old "I like this - I don't like that and I don't understand why you like that" but in fandom context.
Anyway, so much is a given but I got to thinking about smaller fandoms and how this doesn't necessarily apply in them at all. If the fandom is small, then the fandom's main focus - I like to think - is canon and people usually enjoy what is in focus in canon. If Harry Potter was a small fandom, I doubt we'd have maniac DE fans. You look at the fandom HP is, though, and you see them there in massive amounts. There's nothing wrong with that, of course, but it just shows. I guess some of the smaller fandoms I have are mainly doramas. They're OTP-focused and main character-focused and thus it's unlikely that anybody would be into dorama fandom X but not support/be in it for the pairing Y/Z. Small fandoms represent what fandoms always begin with; we like this thing, let's like it together, discuss it, write fic for it, just enjoy it in a group.
Basically I just started thinking about this because with some big fandoms, and people not liking the same things as I do about the canon material, it's almost as if we weren't even in the same fandom to begin with. If somebody watches Heroes and doesn't find Hiro awesome at all - hey such people could exist - then we've not really much in common. If somebody's into Harry Potter for Draco Malfoy and hates on Ronald Weasley, then we could discuss Harry Potter but run into some difficulties aside from the whole general plot. If I start a conversation about HP and the first thing that comes out of the other person's mouth is, "I really wish Ron died", well, that conversation will end there. And I don't mean I'll walk out or anything, it's just that our viewpoints are rather different.
Or say, Green Wing. It's only a semi-big fandom but still, I don't understand the nutty obsessed JRT fangirls (and one of the reasons I defriended the show's LJ comm was because I could no longer skim the annoying "OMG I totally had a drooly fangirly dream about Mac the other time!! *starry eyes*" or the people who think Sue is more hilarious than Alan (I love Sue but she's more random than genuinely hilarious a lot of the time). Alan Statham is comic motherfucking gold. He and Guy remain my two favourite characters, Guy more as a character but Alan as a comic creation.
It also just depends on levels of obsession. If I'm nuts for X and you like Y but are not too fanatical, and can like X too, then we can party even though I might dislike Y. If you however are nuts for Y and will not accept disagreement over Y's awesome, and think X blows horses for a living, and I think Y should die a painful death, then friendly co-existence isn't going to happen; we're better off ignoring each other.
I could get into pseudo-deep thinkings about the whole co-existence thing but I'm running out of steam for this post now.