Can play pretty much anything and be fine in TBC. Only caveat being that the there will be less spots for certain DPS classes, healers and hybrids to fit in your group.
Healing wise most groups would only want one restoration druid, with the rest of your healers being whatever (although shaman healers are generally the best, and will likely be stacked.
Tanking wise most groups will run a single protection warrior and a bear druid. A third tank is rarely needed, but having a flexible paladin that plays all three roles is pretty nice to have, and protection paladins are the best at tanking dungeons and raid trash.
For DPS it's a mixed bag and it 100% moves away from norms that you saw in Classic WoW. Gone are the days of stacking warriors (especially warriors), rogues and mages. Most groups will likely run 1-2 warriors, 1-2 rogues, and 1-2 mages at best. Why? That's all you really need to utilize all the buffs they bring. DPS classes that people typically stack in TBC are hunters and warlocks at the expense of the previously listed classes. Rogues still pump damage towards the end of TBC but are reliant on a melee stacked group (most guilds will have 1), whereas hunters and warlocks work well in most groups and do a crazy amount of damage too while offering more buffs.
Finally that leaves you with hybrid DPS classes (shadow priest, enhancement shaman, elemental shaman, balance druid, feral cat, and retribution paladin). The first two listed are the strongest hybrids and it's not uncommon to run with a 2 shadow priests and 2 enhancement shamans. The other options really aren't that bad, and most groups will fill with a few of those options to round out their raid group.
Obviously there's an optimal raid group in TBC, but shifting from the 'meta' isn't going to hurt your raid group all that much. While a balance druid won't do top tier DPS, the buffs they bring are good enough that the downside to bringing one over another 'pure' DPS class is either neutral, a slight loss, or slightly beneficial. The bottom line here is that rounding out your roster with some hybrid DPS spreads gear out a bit better and that hybrids aren't a colossal overall performance loss for your raid group like they were in Classic WoW (and some of the hybrids are so strong in TBC that it's stupid not to bring them).
What I outlined isn't a free pass to get mad at groups for not taking your retribution paladin or balance druid. There will absolutely be groups out there that will take multiple of these roles, but most are still going to take 0-1 of a lot hybrid classes. A lot better than Classic where most groups take exactly 0 unless it's a friend of yours or they made the guild and are in a leadership position.
The real losers from Classic to TBC transition are DPS warriors, mages and rogues. DPS warriors are literally inferior rogues (you still bring one for buffs) while mages are scuffed warlocks. Rogues are technically good like I mentioned, but you can't really find room for many beyond your 1 melee group.