You want to know what it plays like? It's an old gameplay format similar to Diablo or other more recent top down perspective cooperative shooter. Judging by Valve, I imagine this will be highly polished if not for the months to come.

Plus it has mapping tiles which makes map making either easy to do or randomly generated. Let's wait for updates though.

The video here:

http://store.steampowered.com/video/630/5825