I think SC2 is overrated. I've played the campaign a little and here are my thoughts of it.

1) Diablo 2 styled cinematics are great, especially the Zeratul vs Kerrigan part. Should have more of these.
2) Story is crap
3) I don't like how there can be be some mission you can opt not to do or do later (ie, the order of missions) and how the main story comes after playing some missions. Means the story is in a arbitrary order.
4) I was a Battle.net player on SC but play on normal difficulty and I feel the missions are crap, and easy (maybe because on normal) compared to SC1.
5) The atmosphere SC2 gives you is in no way comparable to SC1. In SC, you get the fear of the Zergs, but in SC2, there're lots of humor, and the battlecruser Hyperion itself is a joke.
6) It eats too much resources, uses over 1 GB of RAM!

More:

7) The SC1 with the cool mouse cursors and great graphics with low PC-requirements where everyone can play lag-free is gone. Replaced with 3D version of the game, which is not excactly so much of an improvement if you ask me. Yeah, I play Ultra VQ, on 1080p resolution.