Got in the Beta for this - Freaking blast. Tho I am getting the Diablo-tired-mouse-finger.
Basically a hack and slash - with tons of different heroes that are upgradable (level and equipment)
You get a squad of three critters and can swap out the active one at any time. (you can have several different squads)
After you finish a campaign mission you get an extra random reward or you may elect to immediately do another mission to gain a second (more powerful reward, but if you die during the second mission you lose all the loot you found in the first mission.
It has online multi player co-op and pvp (with ladder rankings) as well.