TBC was, is, great. It improved many aspects of the game, carried on the lore, brought new content, fixed major class flaws. But something just feels lacking. It just doesn't feel right. Maybe it's the admitted (I wish I could find the quote from a Blue about this) overabundance of purple and pink and fancy technology (that doesn't even explode!). Maybe it's that we're off invading Outland to kill Illidan when he seems mostly content to stay there and deal with the Legion, keeping them occupied, while Arthas only gets stronger in our back yard.
But in WotLK we're coming how. The flaw is fixed. The fight that we left in the Plaguelands is back and I imagine that this time we can finish it. We all know the cold, we were in Winterspring after all. We all know the Plague and Undeath, we have two zones dedicated to it and an entire race caused by it.
This is the expansion for paladins and their fallen brethren: death knights. This is our playground. TBC was for warlocks, demons everywhere to enslave, or work with. WotLK will be the place for death to reign, or end.
I hope this is the time for Blizzard to get back to telling great stories, without even needing words, just the environments.