I had more fun in Vanilla as well. I quit after Cata, since Cata was... CataSTROPHIC! teehee ^_^
Ok seriously though I liked Vanilla better only because it was new, all my friends were playing, it was the first mmo I had ever played, it was an amazing experience exploring that huge open world with all these beautiful colors and magic and shit.
I know the game has changed a TON, but if I were to find myself coming back it would be for a couple reasons.
1) They got rid of flying mounts, and made it so ground mounts were faster. (150-200%) To encourage world PvP.
2) World bosses. Do they have them in MOP? IDK all I know is world bosses were the shit in classic. So much world pvp because of those. And they were quite fun. Any excuse to get out in the real world with some friends and slay a dragon.
"Grammar is important. Capitalization is the difference between helping your Uncle Jack off a horse & helping your uncle jack off a horse."
I played WoW from 2005 to the end of WOTLK in 2010. That was a good solid 5 years of my life.
Vanilla WoW - despite all its fault - was an amazing game when it came out. Leveling the 1st time to 60 was nostalgia in its own.
Burning Crusade was where WoW was at its highest peek and I changed servers and raided in a top 20 guild from the very start all the through all the hard content (BWL, Sunwell) at the end.
WOTLK was a good expansion as well, but its really when they started dumbing down on everything. By the end of WOTLK I just got tired of the same old formula of raiding every night.
I'd say because people queue for EVERYTHING now. There's no real movement. Even the damn raid is 30 feet from either shrine.