DLC serves to add minor things such as new Civs or wonders. What an expansion pack does is introduce features that add an extra dimension to the gameplay. It might also come with a bunch of new Civs/wonders/techs etc, though I would guess (hope) in Civ 5's case that it doesn't and will be cheaper as a result since we've been getting that from DLC already.
For example one of the Civ 4 expansions introduced espionage and corporations, another great generals.
I don't know what a Civ 5 expansion would add. Perhaps religion back into the game, though this is now loosely covered by the Piety SP track.