For me it is the people. In BC it was Purge and the people in that guild that kept me laughing. In WOTLK it was Warbringer, even though I could not raid when they did, I was in the guild for the great, great people. Zazi, need I say more?
Now in Cata I have found the excellent people of LNE. Love, love, love them.
So what is your story, what keeps you on KT?