First off, theres gonna be spoilers in this post. You've been warned.
Anyway, I just finished the game about a half hour ago. I took that long because I wanted to replay the first one so I could get all the story elements fresh into my mind before beginning the new adventure and because I like to play on Insanity difficulty. I uploaded my Paragon infiltrator which was also a female with no relationships in the first game. For the second game I was hitting on Kelly constantly and got pissed when the collectors kidnapped her. I also explored a relationship with Tali (my favorite character from both games) but we didn't end up in bed together, probably due to me playing a female which is a shame (if I'm wrong here please let me know)
I gotta say I like the new combat system, its less run and gun for me and more tactical gunplay/power usage which fits my play style better (especially with the harder difficulties). I felt the story and world weren't as good in this installment though. For one thing the setting just seems smaller even though its on two disks. The story's lack of value comes more from the fact that its the middle child in a trilogy rather then just being worse then the last one but it still had compelling points into it like the human reaper at the end. All that said, I still had fun with it. ME2 showed the darker side of the galaxy which was an interesting change but I don't think I would call it the best change for the game (probably due to my dislike of mafia-esque settings though)
A couple of things that bug me about the setting are thus: I have yet to see any female Krogan, Drell, Turians, Bataran, Elcor, or Volus (unless these races are asexual and I don't know it).
From talking to Tali in the first game (I took her and Liara on all my missions) I was under the impression that the Qaurian's when in the migrant fleet didn't wear suits. They suffered from weak immune systems because of it and occasionally when environmental filters failed on occasion people died. That got turned around into everyone wearing suits even on the migrant fleet which annoyed me
Liara's change annoyed me too, did Shepard dying really make her change so much? I think there is still good in her but shes caught up in underworld politics and trying to out maneuver or out do the shadow broker
I wanted to kill Mordin when I found out he not only worked on but updated the Genophage. From my point of view the Genophage isn't population control as he says but mass murder of a proud and downtrodden people. I made him save the data from his assistant though so maybe we can get it cured in the next game.
When I met Ashley I was ecstatic, thinking she would join the team especially with how close we got in the first game but she just walked off, mad that I didn't contact her even though I tried to through Anderson (my council member of choice). I wanted to yell "I Tried to contact you but Captain Anderson wouldnt let me because I was working with Cerberus!" but that option wasnt there *grumbles*
The most interesting character in the whole game, for me, was Legion. In the first game you run across a few different pieces of evidence that suggest the geth are religious but it turning out to be a small offshoot or cult was an interesting twist that I loved. I just wish he could have been there for more of the game and that he didn't die when we assaulted the collector base (my only casualty too)
Overall it was a fun game and Im looking forward to the next one