Orcs Must Die! [Steam store page]
While not co-op like the sequel, this game has an excellent sense of humor and decently balanced maps. Too bad the humor didn't carry over to Orcs Must Die! 2

This is a fun but easy Diablo style dungeon crawler. It features quick travel via portals and spells, as well as a pet to sell items while you continue killing. Additionally there is a shared chest to transfer valuable items between characters

The casting system is reasonably intuitive, and the gameplay itself is as fun as Torchlight, though the heavy corridor feel of the levels detracts a bit. There is a smattering of story and humor throughout, but I wouldn't call it story-driven.