If we've learned anything from Classic, it's probably that the content is incredibly easy for modern day players. Same will likely go for TBC, even with its pre-nerfed content. TBC will be a bigger challenge but I think we can still expect casual guilds to steamroll through it without problems.
So I've been thinking about what class/spec/build to go with for TBC and as a semi-casual I've come to the conclusion I'm just gonna pick one that is fun to play. Right now I'm leaning towards going full destro Warlock with Shadowfury, because it is just that, a ton of fun, unlike its optimal specs for PvE or PvP. I will welcome the challenge of playing sub-optimally as well.
Will you like me just go with fun spec/builds or will you aim for the optimal ones? I suppose it all depends on how hardcore you intend to be.