Quote from TaylorWell, I'll keep an eye on it. Though, I will explain the rank comment so I don't look like a lunatic. Rank is the name given to a mechanic that's been in games for several decades, particularly arcade games*, that changes the difficulty based on how good you are, or are not. The difference being it was hidden away from the player instead of given a marketing term...
I don't mind this when it's campaign mode, as it makes it possible for friends with different skill levels to play with one another. However, I was pretty irked by the balance choices made for L4D's versus mode, though. The whole point in versus is that it's team v team, and the scoring should reflect the abilities of each team.
For a long time, players thought the health distribution was the same for both teams, but just spread out differently. However, it's not even pseudo-random or indeed remotely fair. The director gives different items to each team depending on how badly hurt they are; this totally ruins the balance on many of the maps. You can be absolutely pounding a survivor team to the point where they're on their last legs. The survivors reach a checkpoint (e.g. panic event health box) and receive 3 medkits, resulting in them carrying on like nothing happened. This is amplified by 'humps' in various maps where some earlier areas are brutal -- one team may get wiped, whereas the other may crawl over the hump but soon get healed up to respectable health levels.
You can verify that this happens by going into spectator mode when dead as infected, then clipping your free roaming camera through the boxes where items are dished out. At the start of the round, each box contains 4 pills max. As the survivors take more damaged, pills turn into medkits. If you need the health kits, they are magicked up. If you don't need the health kits, you don't get them. As a result, it is possible for a team to take a pounding but to score respectably, whereas your team coasts, gets no extras and maybe wins by 100 to 400 points tops depending on the difficulty multiplier. It's possible to dominate on 4 maps, but still lose the finale if they make to the chopper and you get wiped. E.g. you score 400 more per round in rounds 1 to 4 -- their team limps home each time after using about 9000 health kits. Then, on the finale, you get someone punched off a roof by a tank and get wiped... the other team can now have one lucky-ish round and win despite getting dominated in rounds 1 to 4.
I hope they haven't carried this system over to L4D2 because it was really crap.
There was a simple server mod that altered this behaviour for the better. You were given 4 health kits in the safe room, and beyond that you could only find pills. I.e. you'd better save that medkit until you're black and white each time or you're going to lose. It made the game a lot more tough if you took a beating as survivors 
To me, this was the best way forward. It meant if you were playing a bad round, you got punished for it. You didn't magically get over a 'hump' and coast home with loads of health that you were only entitled to because of how badly you were playing.
