Disappointed by:
1: DRM.
2: Deliberately clunky UI.
3: Legal measures to block broadcast 3rd party competitions like KeSPA (which earned them millions of sales in Korea and around the world).
4: Boring units/abilities compared to their potential (changelings, spawn infested terran, mothership).
5: Technical limits little better than WC3 (max map size 256x256 = WC3's max) (max players 12, maybe 16. WC3 was 12), low unit limit (could barely get 3 screens of zerglings which means custom games can't have that many more units than WC3).
6: Achievements (especially those able to be earned in ladder play, meaning instead of just trying to beat you, people will sometimes be mucking around). This isn't WoW top-down.
7: Units not as easily discernible as those in WC3 and SC, especially in battle.
8: Locked camera (in beta at least).
9: Worse damage model than WC3 (too much to remember).
10: Lobby shared with other Blizzard games, so you can chat to your WoW buddies and see their achievements from inside SC2. Ugh.
All in all, its technical difficulty (excluding graphics) is about what I'd expect from a 2nd expansion to WC3 released 1 yr after TFT. Its gameplay seems quite shallow strategically.
However, I do expect it will have a great editor, as WC3's was beautifully designed.
Regardless, it's far from good enough to buy even with a great editor, at least until they strip the DRM.