I've a level 6 Support Specialist, and I laugh at how easy it is for me to still die. If I was ever caught running around on my own with no backup whatsoever, it was because I was out of ammo and running to where I knew I could likely find some or because the area my group decided to defend was foolish and they were going to be wiped out. However, unless the ammunition was in visible range from a sensible defense point, it was often a suicidal attempt at re-arming myself. To be honest, I've seen less of the high level players running solo than I have the low levels (free weekenders...?). I've encountered people starting the game before everyone else is joined up, causing a quick end to that map.
Also, I've seen so many people run off and die on normal mode every wave and proceed to spend the rest of the wave rambling on about how this game sucks, L4D is better, the graphics are crap, etc. The last one was typically remedies by telling the players to change their graphics settings, as they typically were running the default setup. The two before, all I can say is if their gaming "skills" were enough for them to call L4D a "good" game, then I think it is too weak for me.
A freebie to the newbies: STICK TOGETHER! There is no quicker end to a match then a fool running off and getting killed, leaving the remaining team members to deal with the additional wave size his initial presence added.