Mass Effect 2 is a very average game. I wanted to keep quite but every time someone says this will be 'Game of the Year', it makes me a little angry, mainly because it doesn't deserve it. 
 
Problems I have with this game: 
My first and most major problem: Every world you visit feels frozen in time. NPC's don't move or talk to each other, and there is no day/night cycle. Bioware did a good job creating rich sci-fi cities, yet they all feel dead. I think they should really implement some sort of AI system in the next game, so that NPC's actually walk around and interact with the environment. Something similar to The Witcher or Oblivion would go a long way.
 
2. Small environments. Again, Bioware did a good job making awesome looking cities and planets, yet leave you no room to explore. You shouldn't be locked off to one particular area. 
 
3. Graphics. I am playing Mass Effect 2 on full with anti-aliasing forced through my GPU. The game looks good, but has no 'wow' factor. Environments look bland and lack small detail to truly emerse the player into the game. In some locations they even use sprites for NPC's which are far away, and they are noticeably ugly. 
 
I have many other problems with this game including boring characters, voice acting, mining, and just plain laziness by Bioware.
 
Before people start claiming ME2 as potential game of the year, consider the other games being released this year. I personally think these games will be MUCH better. 
 
Halo: Reach
The Witcher 2 
Final Fantasy XIII
Fable 3
Fallout: New Vegas
Brink
Rage
and more...
 
On a closing note, I thought The Witcher was a far more accomplished RPG than Mass Effect 2. For many reasons, mainly being it didn't suffer from the problems I listed above.