Ok I am expecting to be flamed here but...
I'm tired of the fly mounts. I'm tired of no World PVP. I'm fed up of having no sense of achievement with anything on this game.
I miss the Tarren Mill battles, having to run to the summon stone to do a dungeon, challenging dungeons, getting my first mount at level 40 and feeling the sense of achievement when I got it.
I know that Vanilla wasn't perfect, but it was a damn sight more fun and challenging than Cataclysm is today.
There are many, many people who have quit that would return the second Vanilla servers are opened, so what's stopping you Blizz?