Of the two Mass Effects I've played, the first one was the better game. It was also the only good one. I will probably never play ME3 for reasons I'll come to now.

I'm not going to defend the combat system of ME1. That said, I really don't think ME2 improved it at all. I was already bored of cover-based shooters thanks to two Gears of War games, and Mass Effect 2 was nothing if not cover-based shooting. Slow-paced, boring as hell, zero thinking involved. Also, I didn't miss the inventory system of ME1 in particular, but it would have been nice to have a proper choice in weaponry.

The planet-scanning mechanics of ME2 were complete and utter bollocks, so much so that if I hadn't had a save to import (to get a resource boost), I would have played through ME1 all over again to avoid the iridium scanning. The Mako did have wayward handling characteristics and most of the planets/moons/whatever lacked character, but the fact that I could explore them in general gave the world a sense of bigness that ME2 lacks. ME1 felt like cycling through a busy, if fairly uniform city; ME2 felt like being allowed to explore the city however I liked so long as I remained in a bus or within ten feet of a bus stop.

ME1 has a story. ME2 has Commander Shepard fucking around the world, picking up semi-random people so that s/he could do them favours - the "bundle of DLC stuck together and sold as a game" analogy fits perfectly, I think. Then they had the nerve to sell DLC to stack on top of the DLC bundle. No thanks. ME2 is well-written, though, if you care. You shouldn't.

I played both games on PC. From this experience I learned that you should avoid the PC version of ME2 like crotch rot. Here's a short rundown of things that pissed me off:
1) The loading times were atrociously long and frequent - a minute or two to ride the elevator from one deck of the Normandy to another. These weren't disguised as elevators because, presumably, the Normandy isn't very high. Better have a book or a beer or porn nearby.
2) The controls felt sticky, probably because they were programmed with controllers in mind. Fiddling with the options improved nothing. The settings menu is a submenu of "Extras" if my memory serves correctly. Some extras, yeah.
3) The intro cutscene is as close as makes no difference twenty minutes long and it cannot be skipped. Imagine the game crashing thirty seconds into the game, forcing you to restart the game and go do whatever the hell else you have to do for twenty minutes, all the while hoping it was a freak occurrence rather than something the game pulls every time.
4) Remember all those hotkeys in ME1? You know, the ones for the journal, crew members and all that? Poof. Gone. You have to trawl through the root menu every single time, which made me avoid the need to explore all menus for whatever reason. Since this includes the codex, I wound up not knowing half as much about the world of ME2 as ME1.
5) I know I keep banging on about this, but you cannot use double-click. It may not sound like much, but I can tell that double-clicking is a massive convenience that you only begin to appreciate once it's taken away from your life, even briefly, after twenty-three years of happy life and shared memories.

The bullshit that is ME2 made me lose interest in EA in general and BioWare in particular. Don't know, maybe it would have happened sooner or later anyway, but there we go. Well done, lads.