We learn that patch 2.1 will be:
Black Temple is a 25-person raid dungeon located in Shadowmoon Valley.
Explore an ancient site steeped in Warcraft lore
Gain access to more than a hundred new rare and epic items, including tier-6 armor sets
Choose your path through a nonlinear level-70 raid dungeon, as you make your way to...
Face Illidan himself!
So here's a question.. what now. I have to assume tier 4 will be finished with Gruul and Magtheridon, so tier 5 would have to all come from Serpentshrine and Tempest Keep. Those aren't big upgrades for most classes, and we don't really know much about what T6 will be like. So is TBC basically 'finished' with Black Temple? Since we get to face Illidan I would think so. In fact it would make sense if with 2.1 they intend to basically have TBC done, save for the usual bug fixes and class balance issues, so they can start working on the next expansion asap, since they want a 1-year per expansion time frame.