Really don't feel like going into a huge amount of depth on it right now, but I won't join a FC because of a disdain for the very concept of guilds.
I feel they tend to degenerate communities into insular pockets of enforce an arbitrary sense of segregation.
I'd rather the pool of people I meet people from be 'everyone in the game' rather than 'these X people that happen to have the same letters over their head'.

Granted, not joining doesn't really solve the fundamental problem, as the very existence of guilds shifts the community in that direction, but at least I'm not part of the problem. :-\