For the most part, I think ME2 took all the things that were wrong about ME1 and then went to far the other way to fix it. Sure the inventory was incredibly confusing, and the driving was awkward and happened too often, and the places you could visit were too big and too dull to bother exploring it all, but the answer wasnt to remove those things completely!
Compromise is needed.
1)Streamline the inventory, dont remove it. Have a compromise with the upgrading ability of mass effect 1 just with fewer items. We dont need a million types of bullets with 10 different levels each.
2)Combine the driving levels of ME1 with the handling of the Hammerhead. One of my favourite moments of ME1 was driving around the moon of Klendagon and looking up at the great rift. Vistas like that were missing. Mass effect 2 was busy and the views were cluttered. There were no quite open areas. Keep the clutter but bring back the vistas too.
So, have driving in, just make it more interesting than the copied and pasted, barren, rocky worlds, and more open than the winding canyons from the DLC's. Overload was a step in the right direction for that, but it should be expanded upon further.
3)Make places like the citadel bigger again, but not as big as they were in the first game. You are in a massive space station yet can only visit one area?!? Give us 2 slightly bigger places to go per world.
4)Baby Aliens! Children aren't generally in games, but mass effect has a unique opportunity to buck the trend here imo. The combat areas and walky-around shopping areas (for want of a better word) are two different places. Thus the main opposition to having children in games (they will get shot) is negated. Plus we get to see what baby Krogan and Turians and Salariens look like. (Asari and Quarien children probably just look like small humans) Bioware could also be more inventive than just having smaller versions of what is already there. Turians especially could go through some form of metamorphosis during their lives. Mix it up a little.
On a similar note, what do female salarien/turian/krogan look like? Lets see some
5) fewer squad members. I became attached to the ones in ME1, but ME2 has way too many. You dont feel like you get to know them all. Plus, several of them are pretty useless too. (I'm looking at you, Jacob and Thane) whereas in ME1 they were all useful and unique. It seemed strange to me that you have a bunch of people yet only took 2 of them with you on important missions like Horizon. Surely sending more there would of saved more people?
Again on a similar note, if we are going to have lots of squad members, maybe have more missions like the suicide run where you get to have 2 "fireteams". You can still keep the squad of 3 but have your other friends helping you out instead of just chilling on the Normandy.
6)better resource gathering
7)the ability to not take off the normandy when you enter it on a hub world. I just want to upgrade my stuff but I end up flying across the system. Bring back what was there in ME1
8) Bring back the galaxy map from ME1. Flying the ship around yourself seems like a pointless gimmick and not all that realistic. Have the planet selection method from the first game. Fuel and probes were annoying too.
9)more than just one mission where it was possible for a squad mate to die. It made sense in the first game because you didnt have that many but it should have more now you have more team mates.
10) actually have a real enemy. Saren and his geth seemed like a genuine threat and you fought them around every corner. The collectors seemed like a casual foe with most of your time fighting the blue suns and eclipse. There are what? 3 or 4 missions where you fight collectors. You spend more time trying to shut down a rogue VI than you do actually saving the galaxy.