Mass Effect 2 is one of the best games I have played, but it's not without its faults. Most of the negative criticism seems directed towards technical faults, such as audio and movement glitches. Some would also say that the RPG elements such as inventory and character building is dumbed down. The latter is the least valid criticism in my eyes. I'm just going to get this out of the way first.
The character building is condensed, not reduced. The character building in the first game was terrible, a mess of skills, with endless incremental upgrades, placed together to create uncomfortable prioritzation and comparison. Intimidation versus assault rifles vs biotic powers vs class powers? 12 incremental skills of charm? 12 skills of assault rifles or throw where the multiplier increases by decimals for every point, instead of Mass Effect 2's condensed 4-point list where it increases at same rate but without the clicking.
It's one of the little things, the small rewards and feelings that has contributed to successful games like Diablo. You may not have a chance to level up your character after every mission, but Mass Effect 2 does offer a deeper skill system. Mass Effect 2 has more powers and more variations available in a less visually demoralizing menu.
Some skills have been scrapped, for good reasons. Mako is (thankfully) removed, so there's no need for electronics. Hacking is made available for everyone through a decent minigame, goodbye encryption. Weapons skills are moved to upgrades, which encourage exploring and shopping. Health regeneration is replaced with first aid, giving the game better pacing. Intimidate/charm is replaced with paragon/renegade dialogue. This game may not offer a whole new enormous set of skills, but it does offers more balanced and depth to existing and new ones. Thank god for the removal of inventory.
Criticism This was partly addressed in a rev rant I believe. Mass Effect and games in general tend to focus on polarized personalities: evil versus good. The extreme personalities themselves are a problem, but only a part of it. The main problem is that this games poses "big choices" on you, encouraging you to judge them yourself by your own ethics and morals. In Mass Effect, the only way to be rewarded by your choices is to take a path and stick with it. Several of the mission dialogues are only solved by having complete Renegade/Paragon scores. You are, in fact, penalized for doing choices based on your own ethical and moral judgement. This is a problem as no person perfectly fits the renegade/paragon persona in mass effect. I like to perform renegade impulse choices against thugs, such as setting a gas tank on fire to make it explode in their face. I don't like being mean to Mordin and Kelly just because I have to maintain my renegade score, if I want to successfully complete the whole game. This makes the whole game plagued by cognitive dissonance and strips the choices of all moral judgement.
Another concern is that the game tends to appear too designed and structured. Every character has a personal quest with the same trigger source and same time span. At a point in the game, you are basically refreshing a NPC to see if you've hit that trigger yet. When all the mission types have the same duration and trigger, it feels very arcade-ish. The mission debriefing reinforces this feeling. It would be better if the missions were better integrated to appear more seamless in the story.
Another problem, which might be a problem with my self-control and not the game. The choices should be permanent, they should autosave to my character when chosen. If Thane, Mordin or Legion had died I wouldn't think twice about reloading.