Pestilence11
Member
From what I've gathered from dev comments, footage, and impressions, there are three aspects to the progression:
1) Meta-progression spread across playthroughs. Things you do in early playthroughs can have effects on later playthroughs (i.e. something you do results in a boss appearing later)
2) The Isaac-style progression of new weapons and items being added to the pool of loot that you can find in the dungeons
3) The Necrodancer-esque progression where your hub world grows more lively and gains more NPCs, opening up new shops and other elements as you play
I'd like to add, and this is pretty obvious and probably doesn't count, getting better at the game (learning the enemies, what to do/not do) is progression in these games. Dying on the first floor a bunch of times then getting to the point where you don't even get hit, as an example.