L4D is particularly bad for people who just don't want to play as a team. The few times I have tried playing with randoms they have just been terrible. They will run ahead constantly, and get into trouble (smokered, huntered, or trigger a hoard). Then they will bitch and complain about how everyone else sucks. They will also refuse to heal teammates or help out in any way. I've only tried playing with randoms a few time before deciding it was better to stick with friends only.
It does not take long to get to that level (ie: friend only games). But it can back fire at times. One game I joined (was friends only) had two randoms in it (basically) as one of them was one of those "must friend everyone", so you had friends of friends joinging and at that point, setting it to "friends only" does not work.
As to the noobs, I still remember one game were it was me and a steam friend and the other two must have been stream friends as well. We split up on co-op as 2v2 as the others would run forward, trip all the events, get to the end stage, use the health, then pickup the other for a spare, ignoring the us. Got to the point when they got stuck, we just left them as decoyes (ie: they get bombers, throw a pipe or moly at them. near the end, I was on black and white and they did the same health "trick" again. I just went and shot them after getting sick of mentioning it (voice and typing), espically the use health pack when on 90%.
Only play l4d with regular/common steam friends, not any others. Helps to set to expert or hard as well as even noobs are smart enough to avoid those levels (well, the "played an hour" ones are anyway. the "just new" are just clicking buttons.
Had one noob in TF on the weekend, final stange of a cart map (NFI which one now), and they said "pushing the cart is needed to win, that is how to play this map" or some such thing. 20 minutes, 2 stages, and at the last point of the third stage, well, that person had the regulars ripping into him for that.
While there is generally a lot of noobs around, it is just the really noob ones that stand out in these situations.