Fixing the scads of bugs and performance issues that have been in WoW for a long time BEFORE moving on to new stuff with new bugs (like Battlegrounds). They're not so bad individually, but taken together and having been reported and known for so long makes it feel much worse. Yeah, they can have their writers and graphics guys working on stuff for new content now, but put the coders/designers on finishing what they started. I'm guessing that what was shown of WoW at E3 ran pretty smoothly. Give the same level of hardware/infrastructure support to the paying customers.
But, as much as I'd like to see Starcraft 2, if I had to choose between them I'd take "Update Starcraft's graphics engine and re-release it" first, because it's such a perfect game. I'm eager for SC2, it should be a very good to great game if they make it, but chances are that the original will be better.