Well let us use the forums and the community behind it (GameRebels) as the example here.
For some reason what keeps this alive and people sticking together? Similar interests. Where those similar interests come from is complete luck of the draw and down to what happens with personalities, human interaction, etc. Why gaming communities do so well is that you have a wide spectrum of people who are able to join together and maybe find some other people to play with.
Now this can be translated down into the ingame clan level.
You already have the similar interest factor incorporated due to everyone playing the same game, now it just comes down to how well you can work together inside and outside the game (meaning playing the game as a team, and communicating outside of it possibly to further build friendship/teamwork).
Overall it just comes down to how one person will interact with another, and how you can influence relationships through the game and/or the mediums used to play.