It's rated "M" people.
yes that doesn't mean it should have the insane amount of cursing/senseless blood and gore as many other games, but rather have mature, well written themes. If the story clutched to some people so much, whether you didn't like the way a character acted, you loved a character, you hated what a group of people did, or you agree completely and fight for it, the story is well written and definitely not one-dimensional.
Mass Effect 2 is a great game, but as any other person with common sense, it is not perfect. I agree that the main plot line story could have been carried out better, with a bit more answers instead of holes, for example maybe a mission where the colonist from Horizon you saved or Ash/Kaiden have something to say about the Collectors. Much of the story did in fact revolve around the main characters, it was fun for me getting to know them, however in the same token it made the game unbalanced a bit.
in Mass Effect 1 you had 5-6 main story areas and huge decisions to make. many of the missions this time around seem to have a resolution rather than having consequences for your actions or having items from the first game come back to haunt you.
Intro/Freedom's Progress you found out your main enemy in ME1 Eden Prime was similar, however you probably have more dialogue options in the first.
Horizon you fight the collectors, however towards the end you didn't truly have much of a choice when dealing with the ME1 love-interest. Fair Enough, Recruiting Liara in ME1 was kind of forced on you (unless I'm mistaken)
Collector Ship, you find out some major information, however it is here that I believe the choices/consequences become less and/or non-existent, thus making it seem less more epic when dealing with the main storyline.
Afterwards there's Legion and the final battle. I haven't completed most of the side-quest to see how many ME1 things carry over, but I've seen a few, along with the mail you get from your previous side-quest. I think that made my final~perfect run to complete everything in ME1 underwhelming, Conrad is glitched, but he's there, Thorian colonist/Shiala, Rachni Queen mini-talk, I can't think of anything else, but I really didn't feel the changes at all.
I would still rate the game no lower than a 9 out of 10. Graphics were impressive, only "glitchly" graphics I encountered was the time you speak to the council on the citadel. Music vastly improved, cutscenes were more than switching back and forth between the 2 characters speaking, it panned out, zoomed in, different angles etc. Fighting Gameplay was obviously upgraded, interrupt system system is great, upgrades/customizing armor/loyalty mission differences such as Samara's talk with morinth, Garrus assassination, Thane's spying, all were great and had different aspects to them (which I wish were used in other sidequest, however I still haven't done most of them if there are a lot.) The world felt more alive, than bare as it did in the first game. Story, while not as "epic" had its moments. I'm extremely fond of the craziness of the last mission in terms of storyline. My entire team survived, thats including the entire Normandy crew, but I held my breathe that entire mission. It was intense.
I wouldn't have minded if they had all the main story missions they have now, just would have probably liked having at least 5 more story missions... also I loved the customization, but would have liked at least 5 different armors that were buyable, including all the extra chestplates/arm plates etc. have no idea why they have such a minimal amount of that unless I am missing something there as well.