I agree with the above 100% but finding a server with a good bunch of players who consentrate on their class is quite difficult. 9 times out of ten if I find a random server it's full of greedy kill whores who straddle all forms of attack to maximise their kills.People just need to learn the role of their class and stick to it.
Beserkers should sit at the front, taking out clots, gorefasts, stalkers, FPs. Nobody else should be even alongside him. Yes, he'll get the most kills. So what.
Sharpshooters should be aiming for sirens and bloats (both easy one shot kills) primarily, followed by scrakes (still a one shot kill, but beserkers can deal with them easily so less threat), then crawlers (commandos should get these, but putting one HC shot into one makes for easy and useful kills). Only in a situation with none of those types should a sharpshooter even look at another type of zombie.
Firebugs should be igniting then ignoring everything (except FPs) at long range, trying not to fill the team's line of sight with flames too much.
Commandos should focus on crawlers (weak) and bloats (easy headshot) primarily, secondarily sirens (easy headshot too, but more health so easier for an SS) and stalkers (rarely a threat, but can be sometimes if not noticed). They should be completely ignoring FPs and scrakes.
Medics should be sitting behind the beserker, making sure he's alive. When no healing is required he should be picking off weaker zombies that are a threat to his beserker (crawlers, sirens, bloats).
Support specialists are pretty decent all round classes. Normally they should be aiming to cause the maximum possible damage with the least possible ammo. When the **** hits the fan they need to be ready to clear the entire area with grenades and hunting shotgun secondary blasts. What they should NOT be doing is charging out in front of the beserker to kill whore, throwing random grenades that achieve nothing but give the zombies a smokescreen, or trying to snipe with a shotgun. Definitely a class that doesn't have as much of a specific "right" way to play, but plenty of ways people can go wrong.
If everyone sticks to these roles there will be minimal wasted fire, given the nature of the overlapping and complementary roles. A friend and I typically play beserker and sharpshooter respectively, and by understanding our roles can generally (and with little difficulty) hold off a 6 man suicidal wave when the rest of the team (nigh inevitably) dies.
Personally, if I see someone chasing down a potential kill with a katana I don't rape them of a chance to improve their beserker level by shooting the zombie before they get to it. If a player is taking care of a few zombies and has them under control I leave him to it and search out other zombies. I wish players would afford me the same courtesies but unfortunately this kind of behavior appears to be the norm.
With regards to team work; I think it's important to stick together as a team but don't all shoot in the same direction FFS.