Mass Effect 2 plays like a potato when compared to the final piece of the trilogy. Mass Effect, too, though gameplay wise the second and third game are much closer to each other than the first one is to either of them.
But it's incredible how much they improved things for Mass Effect 3. Combat rolls, smoother cover interaction, over-cover grabs, heavy melee, off-cooldown powers, multiple power combos, and even such a minor thing as hit indicator make the third game vastly superior to ME and ME2.
Like with Deus Ex, it started to bother me that I hadn't beaten the previous games on the hardest difficulty setting. And so I started a new complete playthrough as Vanguard since Biotic Charge is so much fun in ME3... and obviously not as fun in ME as the game is missing half of the class signature powers, like Charge, the other two have.
I quickly gave up on playing on Insanity. There was just too much frustration involved. The game is very difficult on Insanity. And shotguns are really bad till you get the spectre ones at level 50+. Also, running up to people to blow their faces off often leads you to just whacking them with your gun instead, thanks to having melee and firing in the same button. Good thing they changed that.
Even though I found myself doing things pretty much the same, I ended up finishing the game anyway. I did, however, pick the correct dialogue choices this time to make Nihlus shoot himself at the end of the game, and thus letting me skip the first phase of the fight. In ME3 it isn't as rewarding because you still get a renegade interrupt to shoot The Illusive Man yourself if you can't or don't pick the paragon option. And there is no boss fight either.
In ME2, my Vanguard Shep finally got her charge but I failed to finish the game on Insanity and played on Normal again. (Maybe Hard would've been better; on Normal everything dies slightly too fast.) Biotic Charge is so much riskier in the second game as you don't have Nova or any other power for that matter to synergy with it due to having the global cooldown affect every single power. Your options after a charge are to fire your gun or melee, which doesn't really work against multiple opponents.
And then there's the low health screen effect that is just obnoxious in Mass Effect 2. It's quite obscuring in ME3 too when you're at half health bar, but in the second game it's like a damn vampire from True Blood had splattered all over your screen. And the worst thing is that it doesn't go away if you recharge your shields with a some power like, say Biotic Charge. You always have to wait for the duration to see something again (unlike in ME3).
Mass Effect 3 singleplayer as Vanguard on the other hand was simply fun. Especially now since they patched tech detonations to not require enemy death. It's just constant explosions. Not that Vanguard really needs them with Nova and Charge.
And of course I then had to go back to Mass Effect 2 again to beat it on Insanity. Infiltrator is evidently thought as one of the best classes. The weapon damage bonuses were indeed handy and I did beat the game, but I'd say the Sentinel had it easier when I did my fourth playthrough afterwards. Getting used to the time dilation with sniper rifle scopes sure took some time after playing the multiplayer of ME3 where time dilation doesn't exist. Especially with semiautomatic sniper rifles the slowdown can be rather confusing.
When using the Infiltrator's signature power, Tactical Cloak, you can notice how the enemies prefer targeting Shepard over her squad mates. I swear every enemy turns their SMG spam towards you the moment your cloak runs out. It's a good thing, though, as the suicidal squad mate AI would not last very long otherwise. Miranda constantly runs into to the open and climbs on top of things to get killed for whatever reason.
Miranda is the squad mate, though. Her two first powers are her best powers (at least on Insanity), meaning that you don't have to hurry into her loyalty mission nor waste points on a poor power. I don't think that you even have to get two ranks in her Overload to unlock Warp. A good thing they got rid off that unlock system for ME3; it really limited using the full potential of your companions.
Infiltrator was also my class of choice for the Insanity run of Mass Effect I had to do then. I had barely touched sniper rifles earlier because they are very challenging to use if you haven't put points in the skill. Although, even when fully maxed, you still have to shoot from cover or at least crouch to make sniper rifles stable.
I don't know if there is enemy scaling in ME. My experience suggest that there isn't, at least not much, and that's probably the better way. It makes the early game amazingly hard, though. I had to reload a dozen times when two (2) turian assassins kept owning me on my first Citadel visit. And then I made the mistake of going to get Liara right away. The last battle against the Krogan Battlemaster was impossible without proper biotics (damn Kaidan kept dying all the time). Finally I was able to beat the bastard by getting him stuck behind one cover.
Liara stayed as my trusted squad mate for the rest of the game. Singularity and Lift are simply a must as enemies will otherwise effortlessly run you down with their mass charges. At one point I had at least eight husks spinning around in one Singularity.
Another hilarious moment was when on some random planet I was fighting a group of mercenaries and one of them got Lifted so high and far that I had to re-embark the Mako to find where he went. Amazingly enough the mercenary survived the fall when Lift ended. He could've gone inactive for being too far, though.
But yeah, I think I have pretty much mastered the Mass Effect trilogy now. I even maxed the last new ultra rares added to ME3's multiplayer. I doubt I will touch the games any time soon. Well, maybe an occasional MP match, though now the lack of any kind of progress makes it notably less interesting for me.