Hi Sasquatch,
I've been in the same clan the last 7 years {DoWUK} , they've been going 10 years I think, having our 10th celebration this year.
http://dowuk.co.uk/news.php
Most people who are currently there have been so for a number of years, as we don't just let anyone in.
It has been mostly for FPS, starting with MOHAA, through COD, COD2, COD4 etc and now and again taking in some other games that some of us play.
(which we did/do play in leagues)
We did set up DoWUK guilds when we played some Guild Wars and LOTRO , but we found that the people who joined weren't as active on our forums or on ventrillo as our longterm members.
As with MMO communities are normally ingame things, whereas for FPS it's nice to have a forum etc to go back to after.
Clans /Guilds should have a charter or ruleset so as filter applicants etc, else sometimes it can get diluted.
You will see communites like Lotus , with 1000's of member accross multiple games where nobody knows anyone.
I can safely name all 20+ in my clan by name, real name and voice.
They are an excellent social forum outside of actually playing too, as you come to meet different people and form real friendships.
Most of the lads are in England, but 1 or 2 are from abroad.
Each year we all meet up for a weekend at a central location though for pints and whatever, and it's never weird, as we know each other like normal friends.
As far as downsides go, now and again you can get someone in your clan who totally upsets the balance of things, and he has to be kicked.
For this reason we have implemented the rule where any new member has a 2 week trial and afterwards has to achieve a 70% pass rate on a forum vote of members.
There only negative, I wouldn't call it bad experience, was before I joined DoWUK after my original clan split up, I wanted to move to a more competitive clan in the league. After a few weeks I realised the "fun" aspect was missing, as it was all about practicing, tactics and matches....there was no late night chats or fun games of random stuff like Starcraft or Civ5 etc....so I left them and found my current clan a few weeks later.