I just rented L4d2 and have been playing tons of survival. In my opinion I think Bungie should try to emulate the way Valve does their survival.
The problem I had with firefight was that it took so damn long for one session. I much prefer shorter sessions where I can analyze my mistakes and develop strats quicker. In L4d2 the shorter sessions allow for teamwork to develop and it also makes matchmaking survival possible. Especially when someone quits and it auto AI's their character.
Its really a buzzkill spending 3 hours on firefight only to get stuck or just have something unlucky happen.
This is just a suggestion/opinion.
Thoughts?