Ah good coincidence OP, I'm replaying ME1 myself, I'm just starting Illos in my current game, and I have completed all Assignments prior to that.
Well honestly during the first couple of hours on Eden Prime and at the Citadel afterwards I really felt like I was playing a much older game (I.E. felt like I was playing something made in 2002 rather than 2007), and it wasn't due to the graphics but more due to the fact that the environments are so bland and generic, less interesting than what I could remember (Citadel and Noveria mostly, being the "biggest" main locations, NPCs sitting at the same place all the time, barely walking around, barely any conversations between them going on as you walk by, not much ambient music or sounds, boring color palettes... doesn't feel like a living, breathing world (or rather, "map", if anything). But I kept going because basically I had no choice, I wanted to create a new Shepard from zero, and import that over to ME2. Unfortunately BioWare hasn't released any official DLC or software that would allow us to just check a few things in relation to story decisions and create a saved game file in a matter of a few minutes.
What I thought was always annoying in ME1 is the inventory GUI, and how big it is, and how unorganized it is. For example I can't filter the inventory. It filters by itself from higher power to lower and that's it. I can't just open an inventory page in which only my weapons would show up, or a page in which only my weapons upgrades or armor upgrades would show up, so I have to manually scroll up and down the list of sometimes more than a hundred of items (limit being 150 items from what I can remember) to figure out what I want to keep, or sell, or turn in Omni Gel. That's one annoyance.
Another thing I don't like in ME1, game-play wise, is the camera issues I often have while under cover. Sometimes I would be under-cover and crouched, and I need to move left just a bit so that I can come out of cover and fire, and then go back to cover. Instead when I move left (or right) from cover just to fire a few shots the camera (sometimes, not always, hence being an annoyance rather than a bug) simply remains exactly where I was while under-cover, so that my Shepard goes out of cover, shoots, but the camera stays put at the original position, doesn't move and so I can't properly target my enemies, basically (visually) it looks as if the camera was looking at my Shepard's back and not moving to the left (or right) with Shepard, nor even zooming in when I do (while out of cover). That issue (and a few more, related to the camera) does not happen often... I'd say maybe 4 or 5 times out of 10 fight scenes but really... what a problem that is when it does happen.
Something else (why not, while I'm at it), is the terrain on most (like 8 out of 10) planets for generic Assignments (and even some that have to do with more important ones, like the five planets you have to go on when it's time to stop the Geth Incursions, those could have been more polished and more importantly... smaller). Additionally, not just the terrain, but the actual size of the maps on such planets, why so big? There's so few things to look for. We either have to enter one or two generic (copy-pasted) facilities/bases/mines, or kill maybe one Thresher Maw, or perhaps collecting two or three mineral deposits. I mean... they could have COMPRESSED all of that in say... 50% of the map's actual sizes. But beyond their size, the issue remains the terrain itself.
The Mako is NOT the problem, remember how the Mako performs on Illos or Virmire? That's right, there's no issues. The problem comes when you have to somehow drive and get pass all sorts of supposedly naturally-formed geometrical shapes of all kinds from the most pointy triangles to the most obvious rectangles, down to the most convex or concave of all the mountains you could dream of... THAT is the problem. There's only a handful of planets on which SOME part of the terrain was seemingly polished FOR the Mako to drive on more calmly, such as on Luna (for the most part), or on the X57 asteroid (Bring Down the Sky DLC), otherwise it IS hell to try to get to those locations for most Assignments.
Another thing that is lacking in ME1 is the A.I. (also the case in ME2 but it's less obvious since ME2 is even more of a cover-system based game than ME1 can be, so the A.I. just basically does what you also do 90% of the time, cover, come up, shoot, back to cover, big demands in A.I. routines right there huh?). In particular the abysmal A.I. from your two party members. But then again... the enemies' A.I. isn't particularly any better so... no need for your party members to be better than them, right? Seriously though, it's horrible, not to mention the less-than-efficient use of automatically-chosen powers (the ones to be used) by your party members (when power usage is set to "Active" in the game's options). How many times did I want Liara to use Throw but she instead just stood still taking fire on her Barriers that I myself had manually told her to use? I can't count. How many times did I want Tali to use Marksman but she instead somehow thought that using Hacking was a better idea? I don't get it. At least I have FASTER (not necessarily "better" per se) control over my party members in ME2... so their relative lack of A.I. isn't that much of a problem.
Last, but not least, the actual Assignments, most of them (not ALL of them, I want to be precise here) are boring me to tears. And that, coupled with the copy-pasted mines, outposts, facilities... it's just too much at times but I have to keep going! I always just remind myself that it'll get better later on (main missions). The ONLY two Assignments I actually enjoy doing (except when it comes to driving on sky-scrapper-high spikes with the Mako to actually GET to their locations once on the planets) are the ones about Cerberus and the Geth Incursions... that's it. The others are just plain boring, but I do them anyway heh, just because I am completionist in games like that, leaving ONE undone mission that I could have done is plain unthinkable.
Now, with all that said, ME1's problems don't dethrone it for me, it's still a superb game overall. In fact, I really like the Virmire mission and I found myself on the edge of my seat once more while doing it. It's certainly one of the most polished missions of the whole game. In fact the more I play it lately and the more I'm lead to believe that (I think) the devs were really adding up the details and the polish towards the last missions of the game. From the moment that the Normandy is grounded at the Citadel to the following events, down to Virmire and the pursuit of Saren up to the revelations from the hologram and then ultimately to the Conduit... I mean it just. won't. stop! It's just frantic from that point and it really feels compelling and exiting, to me anyway, still to this day.
As for ME2... there's more to dislike in ME2 than there is to dislike in ME1, for me at least. I won't name everything but I'll limit myself in saying that what I dislike the most in ME2 is the story and the actual usefulness of ANY of the events unfolding in ME2. That is in terms of relevance and importance within the whole scheme of events that should have otherwise (in my mind) unfolded after ME1. Instead (as someone else mentioned) when ME2 ends... you find yourself back to square one (I.E. not any better in story progression than you were at the end of ME1), with in fact even more questions at the end of ME2 then I had when ME1 started.