I replayed ME1 and yeah it's way harder than ME2, especially at the start. Despite all the flaws (the Mako, the endless elevator rides, the rather dumb AI, ...) I still found it very enjoyable. I liked the story better in ME1 as I found the main story in ME2 to be really short and straightforward (though all the sidemissions and especially the recruitment and loyalty missions lengthen the game considerably while also providing lots of background info). Also found ME2 to be more predictable, run into a large room with lots of cover and you know a firefight will follow, thought the fighting is very enjoyable

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Dunno if you tried the free DLC (Bring Down the Sky), but it's incredibly hard, though it's rather well done and gives more satisfaction than ME2 which I found rather easy (save for a few fights).
Haven't had any issues while playing on Windows 7, but I did play full screen.
The citadel was good, though it does drag on a bit and there's loads of backtracking involved (if you want to complete all the missions at least), but at least it feels suitably big. The ending is really cool and so is one mission near the end (you're told about it in ME2, but it doesn't have the same impact).
They're both great games though ME2 really is more streamlined and polished (to the point of being a bit on the easy side imo). Storywise ME1 is much better imo, but ME2 has better gameplay and the recruitment & loyalty missions are really great at providing loads of background information and thus fleshen out the ME universe.
I spent way too much time scanning planets in ME2, but at least it's not as infuriating as hobbling around in the Mako trying to find your way up a steep hill

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Enemy AI in ME1 is indeed rather silly in that they just do their thing untill you get 1 shot off at which time they usually just rush in. I found that having at least 1 biotic for crowd control helps a ton, so I usually had Kaiden + Ashley in my party until the end where I switched out Kaiden for Liara and sometimes took Tali as well (my pistol skill was good enough to solo kill the mobs while they were kept busy by the AI teammates). I also played as Infiltrator (specialized to Commando) and only had some trouble at the start when weapon skills are low. The sniper rifles are indeed pretty useless, only good for the opening shot which is a shame.
I don't remember getting 1-shot that often, though enemy snipers are deadly and there's quite a few missions where you get rushed as soon as you open the door, but those don't happen too often and mostly in the side missions.