Heh, where to start. I just finished the game, and I have thoughts aplenty. I could honestly just copy + paste paragraphs from what other people have said since most of my thoughts have been covered, but I'll just go through all of it bit by bit. Be warned, it's long.
To clarify, I do like the first game quite a bit, though it has a laundry list of flaws: the poor shooter mechanics, poor writing and voice acting in places, insultingly bad side missions/planets, horrible inventory system, etc.
For starters, I went through the game with a Paragon male character from the first game, most plot decisions where made paragon (saved Council, etc.). I think I'd like to go through the game again with my female character save, since honestly, a number of my complaints about writing and voice acting might disappear. The female voice acting for Shepard in the first game was *miles* better than the male voice acting, and I can only assume that will be true in the second game. It's tough to enjoy a game when you want to beat the main character over the head.
Generally speaking, I liked Mass Effect 2 quite a bit more than the first game. Things that were good: the general elimination of many annoying parts was welcome, the writing was certainly a lot better (relatively speaking, though), and was more fun to play.
Frankly, for the first 35 hours (finished in 38), I thought the second game had a better story. And then the endgame happened. ME2 left me with a very sharp and profound feeling of "That's it?" at the end of the game. I really feel like after destroying/saving the Collector station, that the Illusive Man would've been the final villain for the game. Cerebrus was the enemy in the first, and even if you're on-side with them in this game, you still never trust TIM. I really thought they were going to pull a Melissan from Throne of Bhaal and have TIM as the final villain of the game, but...no. Instead it's some GIANT TERMINATOR-esque REAPER.
Because I feel the suicide mission was the last mission and that it was so very short, the entire flow of the game's story was ruined for me, since so much of the time is spent building your team (which I loved doing), that the plot is minuscule in comparison. The first game, you assembled your team, and then had a ton of plot to accomplish and an endgame that took a few hours. In this, you occasionally take plot breaks while you assemble the squad, and then use the full complement for a mission that lasts an hour. Deflating and pointless.
The NPCs in ME2 are a tremendous step forward from the first game, though personally, they're still not up to the quality of the Dragon Age NPCs, which were some of the best in years. (That's the last and only comparison to DA:O, though. They're entirely different games in different settings and genres for different audiences.) I liked how all of the ones from the first game were included in different ways, and really liked how Tali and Garrus joined you and were more fleshed out. Other characters I liked were Thane, Miranda, Jack (surprisingly), and most especially Mordin.
The loyalty mechanic worked well to give you quests and a chance to make each character shine, and usually, it worked well. The only quibble I had was that with some, the romance got in the way of furthering character development. For example, my PC had zero sexual interest in Jack, but I still wanted to talk to her after her loyalty mission to start getting her into a more stable and open mental place. Or have her reject my therapist-in-command routine. Either. Just SOME continuation.
Generally, the NPCs were better written, had superior voice acting, and were more interesting than the NPCs of the first game. The two with potential were fleshed out in the second game, Wrex (the best from the first game) has a memorable appearance, and even Kaiden and Liara had some good moments in the second game, overcoming their blandness.
Which leads me to something that's been mentioned regarding the romances. I really hope I can keep the romanced character in the second game instead of the one from the first. From the first game, Kaiden is bland, Liara suffers from some pretty bad writing and voice acting, and I just personally have a strong, irrational hatred of Ashley. So, really, I'd rather continue any romance in the second game.
The combat is better. Period. No one will argue that.
I liked SOME of the "streamlining", but it did go too far and really did seem dumbed down. The lack of skills and ways to build a character was unfortunate, and the tier system fell apart for me at higher levels because I was waiting for four levels before I could upgrade a single skill.
I liked the inital armour customization, but that you couldn't do it with your followers was annoying. The non-removable helmets from the DLC armour meant I didn't use them once. That Miranda only had an air mask on WHEN WE WERE IN A HARD VACUUM bugged me.
Likewise with weapons, I didn't like the lack of options. One weapon and then finding one more? I liked having ammo powers as powers instead of upgrades, and the research mechanic was good enough that ME2's "inventory" system is still superior to the first game. Still, I liked the part of the world-building in the first game where they introduced the differences between a lot of the corporations and manufacturers, personally. Finally, I liked how each NPC only got two weapons. Far more clean and better than the first game.
The hacking/bypass minigames were much better. Nothing really to say about it, they were just a lot better.
I missed having a big place to explore, like the Citadel in the first game. ME2 had numerous medium-sized areas (Omega, Illium, the Citadel), but no BIG places like the Citadel where you could spend a while walking around and exploring.
By eliminating the stupid side planets from the first game, in turn it meant far less side quests in ME2, and while that's a little preferable, still a little disappointing.
Finding materials on planets was tedious, but being the unabashed astronomy/sci-fi geek that I am, I still loved exploring each star system and reading the brief history and description of the planets. One thing the first game was excellent at was world-building, and this continued the trend. The planet you find the reaper near? The one with the huge valley from a very ancient mass accelerator round? I remember that planet from the first game.
Voice acting was mostly top notch.
Writing and dialogue was less ham-fisted, but suffered in places. The writing was much "less bad" than the first game, but I'm not sure I can call it "good" writing. As Buck mentioned, this game didn't feature any conversation on the level that your conversations with Sovereign or Vigil (my fav. moments of the series, really) were at. The Illusive Man was good, but since his character never really went anywhere...
Generally liked a lot of the game, though.
Can't think of much else for the moment. Told ya I had a lot to say.
If nothing we do matters, then all that matters is what we do.