This is a change across all versions. In ME1 as you say Shepard automatically melee’d if an enemy was within range, whether you wanted it or not - this was obviously bad in a lot of cases because guns (esp. shotguns) do way more damage than melee (for Shepard). So now you actually have a button for it in all three games and can fire at any range.
The improvements to ME1 are pretty huge. The guns aim/drift/spread mechanics now seem to be able the same as if you had something like 50-75% of the max points in them - i.e. they’re highly accurate and feel like Shepard is actually an N7 Soldier instead of a drunk hillbilly firing their AR one-handed because she’s got a can of beer in the other. Putting points in still ups damage and accuracy a bit, but I found myself feeling a lot less need to put points in weapons, because as it turns out, the main motivating factor in ME1 was the complete inaccuracy/massive drift. I think a lot of people won’t even notice, because by the end of ME1, or on an NG+ playthrough, you’d have accuracy like this, and they probably remember it that way, but as someone who played ME1 from scratch a couple of years ago, it’s really noticeable.
The Mako handling is massively improved, too. There’s an option for “original mode” handling, too, I believe, but by default, it now drives a lot more like it LOOKS like it would drive, rather than like it’s literally made of balloons. It rolls up and down most terrain a lot better. It’s still pretty light (which in-canon is actually a thing - it has an ME generator which angles against gravity or something according to one of the codexes), and still easy to flip when going downhill, but it’s not like, completely routine now.
They’ve tweaked a lot of small stuff about the driving like how the boosts work and the whole thing just feels a lot better. Then to spite you they’ve made all the fights with those ghastly space worms a lot harder lol - but also fairer - now you can see dust coming from where the worm is when its submerged, so you won’t get one-shot by it coming up as much (and it chases you at about exactly the max speed of the Mako which is hilarious/terrifying).
You also get full XP in the Mako, so no more “Gotta get out to finish these guys off”, which like, is definitely an improvement, but it also means you pretty much never get out of the Mako unless you absolutely have to, and there are some places which I remember as super-atmospheric when out of the Mako. Oh well.
Overall the graphics improvements are a big win. There’s a cost - a lot of previously atmospheric places no longer have quite as oppressive/overbearing an atmosphere, but overall, there are so many places which look vastly better, and so many details I’d never noticed before that I’d say it was very successful. I do hope people do some mods to punch up the atmosphere a bit in a couple of places. HDR is interesting - I have a really shitty version of HDR on my monitor (so bad it wasn’t even advertised as a feature lol), buuuuuut, the light-dark thing really does actually work really well in some situations. I’m generally running with it off, because the shitty version I have severely mutes the colours, but it makes some places, esp. on the Citadel, look pretty amazing.
Mostly though, despite enjoying ME1 a lot, and seeing a lot of minor improvements everywhere (even things like walls being removed because there’s no need to block sightlines to prevent an Xbox 360 from being overtaxed anymore lol), I’m kind of hurrying through a lot of it to get to ME2, which is what I most want to re-play, even though I know the changes are going to be much smaller.
I was able to re-create my Shepard pretty well, she still looks permanently displeased lol:
Possibly at how “Pantene Pro-V” her hair looks.
Sadly the photo-mode ones are BMPs which I’m too lazy to convert and won’t upload here. Photo mode has the inexplicably dumb decision (see in other photo modes too) to not auto-hide the photo mode UI when you take a photo, which is worth noting. Also there’s no key bound to it, it’s a menu option, which feels odd but makes sense when dealing with consoles I guess.