It was good. TBC fixed a lot of the original issues with vanilla and just made the game overall better. Dungeon and Raid design improved by a lot. Kara is still one of my raids of all time.
Class design is loads better. Most specs are viable and I still love the support class design of TBC.
PvP is much better and had its own gearing progression. Most specs were viable in some way and more fleshed out.
It's not perfect but TBC was a huge step up from vanilla in terms of design.