Burning Crusade's effect on Blizzard's game was gigantic-- player who'd left came back in droves, and guilds and players who hadn't been playing together for a while all of a sudden found themselves online almost every night. And even out of the game, it had a huge impact-- Wowhead, the argument could be made, rode the wave of Burning Crusade, and it led them to their sale.
So the question now is: will WotLK have the same effect? HC says no-- he says Hero classes as hyped were much cooler than the way Death Knights are being implemented, and that the next 10 levels doesn't appeal to him. They do appeal to his father, he says, and there's no question that lots of players are interested in the next expansion. Who wouldn't want to fight alongside and/or against Arthas? This expansion might get players to connect with this universe in a way they haven't since they played Warcraft III.
Or it might be too late. Even Blizzard realizes the player base is headed for a rough patch-- they've started the Scroll of Resurrection program to bring straying players back with bribes. The game itself is far from over-- there is a significant core audience who still love this game (including myself), and can't wait to see WotLK. But the fact is that WoW may have peaked with BC. Even if there is a nice peak coming again with Wrath of the Lich King, all indications are that it won't be as high as the first expansion went.