Mmmm, StarCraft II.
I think I understand why they're not trying to be super innovative. They know that the last system worked, and they're hoping to try to keep a system very similar to that. They've added things like destructible rocks, Xel'Naga watchtowers, and "macro" abilities like the Queen's inject larva, the Protoss Nexus's Chronoboost, and the Terran's plethora of abilities at their upgraded Command Center. While these don't drastically change the game, they are changes.
I don't necessarily see that as a bad thing. WarCraft II didn't introduce a hell of a lot more than WarCraft: Orcs vs. Humans had, and I still enjoyed it. The main differences really were the inclusion of the new resource Oil, naval and aerial combat, and hero units. WarCraft III continued on with this and added the ability to level up your heroes, give them items, and even revive them if they died (as well as other gameplay changes, like 'neutral hostile' creeps you had to fight on the map to get access to gold mines, neutral shops, fountains of healing/mana, etcetera).
I'm guessing that, if we ever see a StarCraft III, we're going to see some more changes to general mechanics. Innovation may not jump out and smack you in the face with Blizzard, but it's there... it's just very subtle.
EDIT: Did I mention I'm totally more into the idea of the campaign than I am the multiplayer? I've played the current multiplayer in the beta, and while fun, it is not the experience I'm looking for. I prefer campaigns and Use Map Settings maps/campaigns. Those are the -real- fun in StarCraft. And now that we have the Galaxy Editor, I'm sure we'll see tons of great new ones come out, as well as remakes of great old ones!