As great a game that Mass Effect 2 is, brevity of the main campaign is a big let down.
The main story of the game plays out over 4 levels that are spread apart by side content, and are pretty short. I'd estimate about 20 minutes to pass each. These levels are well designed but it feels like the game is trying to push through too much story on very little game play time. The rest of the time you'll be building your squad and doing various side quests and exploration. While building and getting to know your squad is crucial to the experience, it does little to add to the main story.
In a game the length and scope of Mass Effect 2, a more fleshed out story involving the main quest would have been welcomed. The overall experience is excellent, it goes into great detail bringing the universe to life.