Ok, to start this off I know I am in minority here.
I dislike Mass Effect 2 with a passion.
I loved Mass Effect even with all its faults, and while I would never expect the sequel to be the same game, I feel ME2 is just not even the same series.
Disclaimer, I am playing ME2 for my 3rd round, I have some bits I like. Any hate mail will just be a pointless battle of opinions.
To address the things which people hated in ME, I agree with the Mako comments whole heartedly. I could cope with the Mako but half the time it felt like pulling teeth.
The inventory system, none stacking items was a little annoying but to be honest I never minded it, installing mods was a simple task and there was enough variety and no real bad choices meaning you could play combat how you wanted.
Combat, it was bland but functional, it got repetitive and sometimes broken but for the most part it worked.
Elevator loading screens... so? The dialogue was entertaining, it didn't take too long to get places and at least you knew when things were broken. You'd prefer just a big blank screen with the word "LOADING" printed on it?
Finally side quests, yes I agree with the changes made there for sure. ME1 side quests were often uninspired.
So now that I have affirmed that I was not a complete zealot for an old favourite lets address ME2 (or as I prefer to call it EA's mass effect spin off).
Ignoring a ridiculous plot start (seriously Shepard should be a pile of ash scattered across several kilometres, not just deep fried).
ME2 is apparently streamlined to make it more shooter based. I'm not sure what they meant to do but this seems to mean removing an in-game inventory, assigning too many tasks to one button and giving you a really infuriating AI to work with.
Everything for your weapons is now either a one purchase upgrade or a power activated using the radial menu, and while that brings it in line with shooters, combine RPG elements and its absolutely infuriating. How is changing your ammunition a power? Why does activating a Biotic power (think jedi force powers for people who haven't played an ME title) then prevent you from activating a technology power until its finished cooling down?
Here's a scenario I'm constantly stuck in, my character has an ammunition power which effects all squad mates, giving them better ammunition for the situation. My ally has a different ammunition power which does the same thing but its slightly less effective. I activate mine before we go into combat and then after 2 seconds of shooting I find I'm firing incendiary rounds against shielded enemies. Oh no I better use my power and get my shield penetrating bullets back, wait now my allies swapped ammunition again, oh gods I'm using fire to take down a tank, oh gods what has gone wrong.
So after an infuriating bit of tedious conflict I turn off the squad AI, meaning they don't use powers any more, I go back into battle and Oh No my ammunitions changed again, only now my allies not using any beneficial abilities.
Some things make sense as an inventory item, Leave them there.
Next topic everyone else has already jumped on so, yes the planet mineral mini-game is absolutely awful in its thumb destroying tedium.
After that, setting. The worlds in ME2 are wonderfully designed and very beautiful but it seems everyone is only interested in one thing now, their genitals. Everywhere I go I bump into groups of people talking about sex, yes people talk about it in the real world but I don't turn a street corner and walk into people having a detailed discussion about their preferred species of mate. I know ME got a lot of hype for having the romance cut scenes but this is ridiculous.
Equipment or the lack there of. You get very little gear in ME2 and to an extent I agree with it, better to trim it down to about 5 distinct varieties of assault rifle instead of 20 similar ones. But thats it, you don't get 5, you get 2. One which is useless and one which is good (unless you get the DLC in which case you have 2 which are useless). Bioware RPGs are normaly filled with variety, but in this game there's nothing of that. Its just another generic shooter in the mass effect universe.
Moral choice is no longer as clear cut, while Shepard needs to do morally ambiguous deeds now it often feels like the game is driving Shepard into one of two personalities. Theres no real room for role playing. Your either a sadistic murderer, or a well trained lap dog.
Characters, I loath most of ME2's new characters. Their are exceptions but for the most part they make me want to use the disk as a whetstone for an axe blade. Jack, Miranda, Kelly, Thane, Jacob, all the Normandy's deck crew. All of them feel like shallow cut outs, with no emotion, depth or real motivation beyond simply following the propaganda posters. Excuse me if I don't want to socialise with someone who can't think for themselves. The remainder are either characters who's ineffability is central to their existence, or who continued from Mass Effect.
Dialogue, where to start? It's either "I will shoot you unless you stop being evil" (and that's the good option), "I'll shoot you then tell you to stop being evil" (that's the renegade option) or finally "Lets talk about cross species relationships again" (with accompanying good and bad versions there of). What I'm saying is that I feel like this isn't a Bioware dialogue driven RPG, but merely manoeuvring down fixed paths more akin to the Japanese interactive novel. And again please stop with the innuendo, I did not need to know the details of Tuerian mating.
Ok I didn't realy want to mention this but now I feel I have to. The story in ME2 is complete garbage. It makes no sense what so ever. The big plot twist concerning the station was about as shocking as dry toast. The other major element concerning the origins of the collectors was good and stopped me using the game manual as bedding for an incontinent mouse. But otherwise its nonsensical, easily foreseen and just as easily derailed.
So that's my ME2 rant. I could think of alot more but I'm tired of writing and no doubt your tired of reading. Again, yes I know I'm in the minority and I think if I hadn't played the first mass effect then I might look more favourable on it (aside from not knowing what's going on). But for me, this was not Mass Effect, this was a mostly functional, but poorly planned and executed shooter with a JRPG style of story telling (with a bit more flexibility admittedly).
Remove the Gears of War influence, the Halo influence and then add in a bit more Bioware style influence and I would love this game.
If you realy press me then I will give you a list of things I can like In ME2, but for the most part I spent as much of it as I could with old characters and ignoring everything around me.