It’s not any one thing or any one memory, it’s more my gut telling me what I once knew for fact - it’s been years since I played the first one (to completion with @ShivaX) and only hours since I last played D:OS2, so it’s really hard to articulate exactly why the sequel is better, I just know it is.
I’m trying to remember the first game, I know the story was kind of dumb (to me) and it was a much “sillier” game in some ways I didn’t appreciate. The story and events/characters in the new one are much more interesting to me, from my recollection.
Mechanically, I really prefer what they have done in the sequel with regards to how armor (physical and magic) block effects to help you avoid spending half of a battle frozen, stunned, or knocked down. Stripping the armor of something is usually a death sentence, because once you lack protection against various effects, that’s gonna be a bad day. In the first game you could just end up stunned before you even got to take a turn, which I remember being super frustrating. In the sequel the satisfaction of finally taking the physical armor off a tough foe and then knocking them down or petrifying them is so good, every time.
I don’t remember the crafting system specifics from the first game, but i hated them. I love crafting in OS2 though, it’s awesome and it’s easy and it’s super useful, all the time.
They added Races like elves and such that have cool abilities I really like. I remember being really excited about that. And the sequel has a LOT more interaction with regards to dialog, like with the races and tags (Mystic, Scholar, etc.) and various ways persuasion skill checks are done now. I also prefer giving each character (all four even) a voice during conversations, and having secondary characters play a role, rather than you playing their role for them. The story elements are done really well here.
I do not miss having to rock/paper/scissors with my co-op partner.
I remember when I first played D:OS2 really liking the skill system a lot better, the variety of skills and skill “schools” and the way you can craft/merge skill books to create new stuff with multi-school requirements is really awesome. The level up progression system is a lot more interesting to me, but I don’t remember the first game’s that well. Looking at old screenshots it seems a lot more simplified and less about diversity in builds, but I can’t really quantify that with an example or anything. I think the skills themselves are just a LOT more interesting and tactically varied/viable.
In general, I think the improvements to combat and mechanics from the first game can’t be stated enough. I can’t imagine not having access to the tools and QOL stuff added in the sequel.