I just finished Mass Effect 1 last night, and it is definitely one of those games, where you don't quite appreciate how much the sequels changed/improved, until you go back and play it, or play one of the sequels immediately afterwards.
Other than the combat, and other general mechanics, the biggest things that I noticed was how noninteractive a lot of the characters can be. On missions, your squadmates very rarely chat with each other, or chime in during decisions to offer their viewpoints. Most of the time, they just end up feeling like nameless, faceless mercenaries, instead of actual characters who are invested in the mission. On the ship, your squadmates only really have something new to say after a main mission - which is fine - but a lot of the extra crew like Presley, and Dr Chakwas, don't have any new lines after your very first interaction.
One thing that really did surprise me though, was how unphased your squad was, after the Vermire mission. Only Ashley/Kaiden have something to say, and everybody else is just business as usual.
But, I do notice that I have been quite critical of the game. At the end of the day, I did really enjoy my time with the ME Remaster. They took a game that I couldn't stand to replay, and polished it up, and changed it just enough, to a point where I can gladly play it, and be happy with the 39h I sunk into it.
The one thing that this game does an excellent job at, is worldbuilding. and setting up the story that the series would continue to tell. I was sold on the universe when I first played ME1, and I was excited to rediscover all of the little tidbits of information that I had forgotten over the years.
The game can certainly feel a little clunky at times, but it is all worth it, for the world.