So I've read up on this now......
The game has shifted pretty significantly from its original design.
Before it was really just a sandbox cat breeding simulator. You had a house, filled it with cats, bred them, mutated them and then sent them out to do mini-games. There were some NPCs to interact with, and according to McMillen, the end goal was basically just earning money to buy furniture for your house. And, of course, appreciate your kitties.
They realized they needed to tighten up the game so it was more of an actual game and less "every idea we had."
After several iterations, they settled on a Final Fantasy Tactics-style turn-based battler.
So now, the shape of the game is thus:
You have a house. You have some kitties. Those kitties have RPG stats, classes, equipment, abilities, they level up and they can die.
Days are passed at your home by having your kitties eat food. And you send a party of 4 kitties on adventures to acquire more food. The game is broken up into "Chapters" much like Binding of Issac. So each run is finite in its length. There's random events and NPCs and other things to break up the battles as well, so it's not completely one note.
After your kitties go on a few adventures they get old and are no longer able to go out. But you can breed them together to birth a new generation of "better" kitties to continue the adventure until you reach the last chapter.
There's a lot of skill, positioning and action synergy in the battlescape, where it appears most of the recent development effort has gone toward.
NGL, I actually kinda wanted a sandbox game about breeding cats more than I wanted a roguelike with turn-based battles. But what they've done so far looks really good. There are over 400 abilities and items and they claim there will be even more by release, surpassing the Binding of Isaac in total counts of "stuff." I'm sure there's a lot of recycled ideas by McMillen there, but he's never made something I didn't like so I'm not complaining too much. I'm sure as the months go on I'll find a deeper appreciation of the new format.