So, without the expansion, it looks like you're going to get a free respec, and of course all the talent changes on the trees-- but you'll still have only 51 points to spend, even though the trees will now go up to 61 points. And you'll still have access to all the content currently in the game-- you'll be able to play Strat, Scholo, ZG, AQ, and even Naxx until your heart's content. And even though you won't be able to play the new races, you'll still get to goggle at them as they run past you, sometimes on their very sweet epic mounts (although you won't see the flying ones, because those will only be in Outland, somewhere you can't go).
What else won't you get? You won't get to learn Jewelcrafting, although you will be able to use Jewelcrafted items and obtain drops. You won't see any of the new dungeons, and you won't be able to level a single XP past level 60.
And then there's some things we just don't know about. All players will be affected by the Honor System changes, but will Arena PVP be only for Burning Crusaders or will everyone get to play in them? And Blizzard hasn't said a single word about how the expansion will be implemented in-game-- will there be a World Event like the Scourge Invasion, and if so, will everyone see it, or only those who buy the expansion? Caverns of Time, we know, will be a world event in the "old" content of Tanaris-- can everyone participate in that or only those with BC?
And I know it boggles the mind, but what about post-expansion patches? Will Blizzard upgrade the entire game, or just the expansion folks? Lots of questions to be answered as we move closer and closer to BC's release date.