I just wrote it on Gamersyde forum, thought I'd paste it here as well.
TBH Mass Effect 1 was already a very streamlined RPG, lacked in content, was poor as a shooter, had broken inventory interface, very lazily done loot (Phantom VI, Eagle III etc)... It was not a perfect game by any means. With ME2, they tried to fix some things either by improving upon them (shooting) or cutting them altogether (the rest). ME2 also had a lot more content. Where ME2 failed was the lack of overarching story, more generic music than ME1 (when I first fired the menu screen in ME1, I was immediately sucked into it, that theme was incredible) and finally fewer epic, cinematic moments like the last two hours in Mass Effect 1, which was one of the greatest moments in gaming (the landing on Ilos until the end of the credits).
Basically Bioware needs to make the story and cinematics as great or greater than ME1 and all other flaws will be forgiven. I'm not sure they can achieve that though, they already revealed that the game will revolve around Bioware's cliche alliance seeking (like in DA:O or ME2), so I'm not getting my hopes up. Mass Effect 1's investigation theme was much more interesting.