I just find that the game is too shallow to warrant more than a few playthroughs of each campaign in coop. Even with the "director" making things somewhat random, every encounter still goes exactly the same.
- I found the director meant each run through was fairly different.
For a coop game to have proper staying power, you ideally need to have players settle into different roles -- that way you get to react to each situation with your 'class' in mind. Having 4 characters that are all the same and a very limited choice in weapons really doesn't allow any flexibility.
I felt the weapons offered enough variety. There really didn't need to be any more. To be fair, on pretty much every fps i ever play, I always find there are LOADS of weapons i never use. The classes are quite obviously close cover (shotguns) long range high damage (hunting rifle) and a mix of both (assault rifle). Then again, I find strict class based games such as TF2 to be pretty annoying really.
Versus runs into the very same problems, but it takes a little longer. Versus was also pretty epically broken for quite some time, so that delayed people properly learning it.
Ultimate L4D strategy: melee, melee, melee, melee, shoot, melee, melee etc. Replace 'melee' with 'run' when faced with a tank.