Starcraft 2 keeps the gameplay familiar, but definitely improved. I couldn't have suggested improvements - perfection.
The three races have really distinguished themselves in how everything looks. Zerg buildings really do look pretty disgusting and slimy, while ol' protoss ships are all enigmatic and shiny and stuff. Then you have the new Firebat who looks so badass. I really do love the new look. Oh and the terrain is just stunning. And now it's dynamic too (if you want it to be)
Sound - The new terran theme isn't as catchy as the old one sadly, but the wings of liberty theme and other more orchestral epic pieces really set the mood for the fight.
Gameplay - I think there's too much to be said here. Basically it's the same but there's lots of new strategies and the game has a totally different feel to it. Smoother gameplay.
Story - blizzard definitely amped up the story factor for II. You see raynor up close and personal (instead of in his teal vulture) and you'll see faces both old and new.
Above all else, it's the multiplayer that makes Blizzard RTS games so awesome. I've only played a few matches so far, but in time the community can only improve. Also those amazing custom maps are going to be coming out eventually - I can't wait! This game is going to last me years. For 60$, it's a damn good deal. Buy it if you liked SC1. Try it if you even remotely like RTSs. 10/10