http://pc.gamespy.com/pc/krater/1221304p1.html(There's a video in the Gamespy article. A direct link to the video on IGN wasn't loading for me.)
In a nutshell: control a small party of three semi-humanoid mutants as they scavenge for lost treasures in the depths of a massive shaft that runs deep underground. It's Diablo-y, with a Baldur's Gate-style party command interface. The character portion of it is focused on creating a well-rounded party to take on the deeper levels of the crater. Characters can be built to be tanks, DPS or healers, or you can hybridize them and try off-beat combinations like No Tank or No Dedicated Healer groups.
Why I think this game might appeal to Bay12: permanence. Every time a character goes down in combat, they get a wound that gives them penalties. (Broken leg, ect...) If they don't receive medical treatment soon, that wound becomes permanent. And of course, they can die. Permanently. There are a
finite amount of scavengers you can hire to join your party, so your characters aren't throw aways. The stat bonuses you choose for them are also permanent, so there's no "ah I'll just respec this guy to suit my needs." Gear can be switched, but most traits are permanent.
The developers are touting a really extensive crafting system too, that apparently can be used to rebuild your character's appearance as well as create items.
The game will also feature some sort of limited co-op, which may or may not be in by release. It'll be released episodically, with the later episodes coming cheaper than the first.