I could pick apart Steam and L4D2 and everything they've done wrong with it.. But I'm not going to.. Why? Because Kavinsky will do it for me.

+1 Kavinsky.

Anyway, back on topic.. L4D2 IBIS Team? Already been done. Me, Hobo, Danger Dan, and usually some random dude from ZM play it from time to time. It isn't something like Pub or ZM on CS:S though. Apart from the Barricade Mod I got working..