From what I've played on Vanilla server, I can mention only two things I liked there more: questing difficulty and lack of streamlining. While still an easy game, you had to actually be careful about who and when you fight, a lot of quests didn't have much direction and no map marks, so you had to read the quest description and try to figure out the rest... Some quests requiring you to fly to another city, only to talk to an NPC there shortly and then fly back - I think these things add to the feeling of the real world. Now it feels more like the game is catering for much less intelligent and demanding players, those who value reflexes over thinking.
Aside from that though... Overall clunkiness of just about everything, extreme immobility of the character, lack of diversity of items, much fewer areas to explore, a lot of annoying mechanics that make the gameplay more tedious than challenging... It seems to me that Vanilla is a very raw product catering for serious gamers, while current WoW is extremely polished quality product catering for a more mainstream crowd. I still much prefer current WoW, from what I have seen.