Lack of story and innovation leads to lack of satisfaction
Although they have polished the excellent concept, you'd have thought after so many years in development, they would have built on it but they've decided to stick to the same concept and sit on safe solid land instead of walking a wonky line in an attempt to bring something new to the RTS genre like Company of Heroes or SupCom did. This isn't necessarily bad but it isn't necessarily good either; if EVERY sequel to every game ever made was just a reiteration of the prequel, the games industry would be heading nowhere fast.
Singleplayer campaign
The story is poorly executed; Blizzard attempted so hard to build a love story between Kerrigan and Raynor when their wasn't one in the first place, from their actions in the first one i'd have said they were friends at most. This is all thrown into a mix of supposed untrustworthy characters who you can't really tell if they're trustworthy or not due to the voice acting being as bland and unentertaining as watching Vespene Gas being harvested.
The ending left me feeling sorely disappointed as it was anti-climatic especially after a massive build up and their lacked any really flow between the characters; Raynor was as blank-faced when he found Kerrigan as he was throughout the entire campaign and he continues to be blank faced even after the betrayal of Tyrus. This may just be me but i would have shown atleast some emotion after finding the girl i'd been attempting to save for so many years and when my supposed friend was working against me all along.
Multiplayer
With the battle.net 2.0 features of chat channels and the whole "map marketplace" missing it really does feel like you aren't actually playing multiplayer at all but playing with bots who happen to use different tactics to bog-standard ones, this is reinforced by the fact that in the league games you're forced into fighting higher ranked players even if you're ranked pretty low and that near enough everyone you come across in games either doesn't like talking or can't speak English (on the EU version anyway).
The lack of a "map marketplace" is a real let down because the maps released with the game become boring fast and it's made me realise how much the custom maps brought to Warcraft 3's staying power, the likes of DotA, Custom hero line wars and Tower Defense brought this on.
All in all, the game has been severely overhyped by high expectations of "Blizzard's never makes bad games" and the fact that "South Korea loves it... so it must be good, right?" as one too many fanboys have told me. This is the first Blizzard game to ever disappoint me and i'm sure it won't be the last now that Bobby Kotick has their balls in a grasp and will be milking that cow for a long time to come.