I've spent some time figuring out how to enhance the third person view of L4D. Since thirdperson requires the server to have sv_cheats enabled all that's left for us at the moment, in multiplayer matches, is the thirdpersonshoulder view. The only problem is that the cross-hair is becoming imprecise due to the camera angle differing to the actual view of the character. So I played around with the console a bit, and I figured I might as well write it down.
To get the cross-hair working as intended I changed the c_thirdpersonshoulder settings; aimdist and offset (For Example: 'c_thirdpersonshoulderoffset'.), also handy come in the height and dist settings to get the camera a little further away from our character, enhancing our actual view of the surroundings. Also noteworthy is the cam_ideallag command, which sets the delay the camera has when following the view of the character.
To make it short, I set the offset to 0 and the aimdist to 320, then I was able to make precise shots over a far distance, which was nearly impossible before. But it's still hindering in gameplay, since half of our screen is overlayed by our character model, so I set dist to 180 and height to 0, and now I have a perfect overview to the surroundings which helps me a lot against zombies running up to me from behind and also provides me with more information on the status of my teammates.
Now there's just a little problem we might encounter when using this camera mode, and those are certain doors and walls. (Edit: Aside from the spawning room of the Runway Finale stage I haven't been able to locate any other such walls yet.)
To avoid this, I simply bound 'firstperson' and 'thirdpersonshoulder' to my keyboard to allow fast switching. (For example: 'bind f2 thirdpersonshoulder') Not the perfect solution, but it works for now. A little downside to third person are the missing shotgun sounds, although personally I don't care about it, it's a bug which will hopefully be fixed soon.
Lastly, when using this camera mode you might consider setting your cam_ideallag to 0 to negate the camera delay when turning around, resulting in enhanced precision.