One of the best post-release surprise additions of the past few years was the Last Stand co-op survival mode that Relic patched into Warhammer 40K: Dawn of War II. Together with two allies, you'd stand against wave after wave of attackers until you were finally overwhelmed. The only complaint I ever heard anybody level against Last Stand was that there wasn't enough of it - there's just one map, and it spits out the same waves of enemies every round (albeit from random directions). The biggest disappointment of the Chaos Rising standalone expansion was that it only added the new Chaos and Tyranid heroes to Last Stand, but in the upcoming Retribution, Relic adds a new hero - the Imperial Guard's Lord General - and a new map that will challenge even hardened Last Stand players.
The first three or four waves of the original map were warmups, and could be survived with barely any player action at all. But in several attempts, I never made it past the fourth wave of the new volcanic-themed map. To be fair, I was playing with the new Lord General, a fragile finesse hero that relies on summoned reinforcements and deployable turrets who I'm a long way from mastering, and I was playing with only one human ally and an AI stand-in (a testing feature that won't be in the final product) but the first waves of Tyranids, Eldar and Imperial Guard knocked me around badly. The first IG trooper grenade volley took me down to half health and sent me scurrying around the map, trying to keep my summoned Guardsmen alive long enough to do damage while taking potshots with my sniper rifle. After failing badly, I swapped out my sniper gear for a saber in the next round...but neglected to swap out my armor for something more suited to a melee fighter and was quickly cut down.
So yeah, it's challenging. Still, Relic expects that the community will reach the 20th wave within hours of the new map going live, in part due to the fact that players won't have to re-level their existing characters when making the switch from Games for Windows Live to Steamworks. The game will automatically import your DoW2/Chaos Rising saved game into Retribution, so you'll be able to pick up right where you left off when Retribution comes out on March 1.