I'll be straight up, I haven't played in about a decade and I'm just dipping my toes back in anticipating the Steam version.
I'm reading through this thread (apologies if I miss things) but the things that stand out to me so far:
1) The default loadout for a new "play now!" embark needs to be better, and include more of the things that it's easy to miss if you don't know what you're doing (e.g. gypsum plaster, dogs, chickens, llamas).
2) I'd really like it if the auto-search-for-an-embark-spot feature included searching for sand (i know high drainage is a proxy but still), high cliffs (elevation variance not just elevation), and surface magma (lava) as options; it's a pain to search for all of those manually.
3) adjustable font size. Some of the text displayed in the steam updates was really hard to read; we're all getting older, larger text is better.
4) Quickfort should really just be incorporated into the base game. (Speaking of, will the same keyboard commands carry over, or will they be changed for the Steam version? I don't want to have to redo all my old quickfort plans).
5) just always start on the first of granite. if you want the option to start on a later date bury that in a menu somewhere. Hitting that screen right at the start is a newbie trap: what the fuck? which day is good? which day is bad? why is it advancing? aaack!
6) Goblin raids and army seiges seem to not happen much any more? Those need to come back, or people aren't going to have a game to play with their game. I guess it's a worldgen issue but that needs significant tweaking so that raids happen with some frequency after the first few years; goblin armies are just the core of the DF experience for me.
7) some difficulty presets would really help the newbie experience (few evil zones, no werebeasts/megabeasts in the first few years of the fort, etc.)
More as I play a bit more deeply again.