I think vanilla was fine, the game has also greatly improved over the course of vanilla. AQ40 and naxxramas were simply great encounter-, design- and gameplay wise. and at least my class, the mage, evolved a lot an was super fun in those raids, rolling ignites anyone? amazing.
I am aware that especially Burning crusade improved WOW a lot in general, but to me at least, there is still the community aspect, and although I completely agree that this is completely subjective to me and my server, I like to think that same server battlegrounds helped a ton to form a great community. that was just destroyed after vanilla. all the people I know in that game, both horde and alliance, I know from vanilla PVP, and boy those were glorious times. best thing I ever did in any game.