I love the content of both pre-BC and TBC (I am one of those that will always consider Vanilla Pre-BC like we did before everyone started calling it Vanilla). Nothing was removed, so in TBC if you missed Naxx, you could bring your 25 man raiding team to it and still see old content.
The lore of both were pretty amazing.
The events in "vanilla" were better than TBC (AQ server progress > Shattered Sun... Select person/few got Scarab Lord and anyone with 1kg got the Shattered Sun title).
PvP Honor system was better in "vanilla" IMO. TBC, you could just farm honor in BGs and use said honor to buy gear. In "vanilla," you needed to be in a cert top % for certain gear. That made it more rewarding.
Title system in BC was a little better. Hand of Adal, Champion of the Naruu when you get attuned to show it off and if you wanted you could switch to your "vanilla" PvP ranked title at will.
Tier tokens > tier drops.
In all, both have their pluses, but both are still enjoyable and I plan to play this server to experience all the content again, the send my priest to the TBC server to experience that as well.