As for ME1 and 2, I think most are in agreement the second was by far the better game. I complain about it, but they are mostly minor things that hopefully Bioware addresses.
What I want from ME3:
- Back to level 60
- More weapons, sorry but 3 per class is just not enough
- More abilities, I liked having 8-10 from ME1, go back to the ME1 style leveling system, which brings me to
- Experience from individual enemies and missions, after each mission only (like ME2) was stupid
- Be able to use more than 1 ability at a time, like ME1
- Better story missions, ME2 was good but the story in ME1 was superior and the missions were funner (Noveria, Feros > Getting companions)
- Flying missions, Halo: Reach proved that you could make badass flying missions in a shooter game, I want more!