By the sound of previews the game is just going to be a graphical improvement on the original with little new in the way of gameplay innovation, just a few new units, shiny 3D look, more units on the battlefield. I'll still get it for the single-player though, Starcraft cut-scenes were awesome.
Um, I think a lot of gameplay changes will be made. First off, nearly all of the new units and even the old units have special abilities of some sort, which is a huge difference. Second, if you have watched the videos in depth, you would no that the unit counters are much more extreme now. 6 Immortals killed 5 siege tanks on only 2 of them died. (the other 4 still had shields up)
There will be more units to choose from... Heck, there could still be another race that hasn't been revealed yet. Don't you think that might just change the gameplay a little bit?