Another aspect of Left 4 Dead which should be considered by all designers is the option for an AI stand-in during multiplayer.
Most gamers acknowledge that gaming takes a backseat to other things, including everyday distractions like phone calls. In most multiplayer games, gameplay cannot be paused; and when gameplay can be paused, at least one person is left sitting idly in boredom. Left 4 Dead allows a player to call an AI substitute, so others can go on playing.
It's not a perfect feature. Any AI is imperfect, and a substitution can mean someone is temporarily playing by himself. But I'd still recommend a similar feature for almost any multiplayer game (excluding MMOs).
I'm enjoying Thanksgiving out of town this week, but hopefully I'll be able to keep posting stuff here 'til I get back. My thanks to God for all our many blessings.