Call of Duty 4 tried something "new" by including a player leveling system that was attached to a complex unlocking system. As you leveled up by killing people and doing a variety of other things, you got experience points and those points fed into your level. A new level meant you got access to new weapons and other gear (including perks).
Initial reactions to this idea were that people who play a lot, and thus hone their skills, would be rewarded with the best weapons and the scrubs of the world would be doomed to a world of hurt. Infinity Ward did a remarkable job balancing the unlock progression so that the super hardcore didn't earn themselves a god-like immunity to everyone else. The first weapon you got was a capable weapon and the best unlockables (such as the Red Dot sight) didn't require weeks of playing (perhaps only an hour or two).
However, given a system where there are levels, some people will figure out a way to game the system and "boost" their progression thru the levels as quickly as possible. The "prestige" mode, by which you give up all of your unlocked goodies after you hit max level, served to encourage this behavior. It's more prestigious (aka, my "epeen" is bigger than yours) to give up all the goodies and still pwn noobs. Ultimate epeen-ness (my apologies for using a non-word and adding suffix to it) in Call of Duty is achieved by hitting the max level in the game and 'going prestige' 10 times. At that point you're done and get a shiny icon that shows everyone in the lobby that your epeen is large and throbbing (and/or that you have no life).
That well designed incentive system, that begs you to keep playing, encourages people to "boost" and people inevitably answer that call.
Now, if you create a system with a reward at the end and then have people that want to circumvent that system, you are obligated to keeps things fair for everyone (even though it's all meaningless). In Call of Duty, and in any other game with this system, the method used to keep things "fair" is to not allow any leveling in private games. Those games would of course be the best method for someone to boost, because they could just invite their friends and grind thru the matrix of levels, unlocks and achievements. This wouldn't be fair to the player base, so XP gains in private games are always disallowed and are instead only allowed in public matches, which are predominately populated by assclowns.
The result is that people usually don't play private games, at least until everyone has unlocked everything and there's no more progress to be had. In Call of Duty 4, the progression system was so long that private games were rare. The same is currently happening in Modern Combat 2 since it has the same leveling structure. Battlefield Bad Company 2 appears that it will follow the same pattern.
The reason why this is bad is that the general public ("GP") on Xbox Live is the internet's version of Mos Eisley ("a wretched hive of scum and villany"). Many online gaming communities have been formed due to this exact issue. A 30something like myself doesn't want to play with 15 year old kids and listen to them pretend to be tough and whine about their homework. It gets much worse than that too. I'd rather play with men (and women, and not sexually harass them) closer to my own age and closer to my own tolerances for bullshit. The older people get the better they are able to divorce their sense of self worth to their in-game icons, the inverse is also true.
Being forced to play in public matches will especially hurt when Battlefield Bad Company 2 comes out tomorrow. More than any other game I've played, teamwork in BFBC2 is an essential part of success and having fun. The reason why I play video games is to have fun. It's as simple as that, and playing a game with random GP assclowns is rarely fun, especially when the game itself is so focused on teamwork (which is a rare commodity when playing with random people). But, if I want to unlock all of the weapons and attachments, then I have to play public matches.
My question for the industry and community is, why do we care that people will "boost" themselves? I don't care that some kid spent his weekend with a friend getting himself an icon (that is only shown in the game lobby). What I do care about is having fun when I play a video game, and the GP usually ruins that, so I want to avoid them.
The status quo of only allowing leveling/unlocking in public games hurts the people who want to play private games. The irony is that private games don't get formed because of the boosting issue that that was created by having the leveling/unlocking system in the first place. Again, why do we care that some people will boost? Because he got the achievement for stabbing 10 people in one game? I DON'T CARE. I want to play with my friends and not be penalized for it. EA and Infinity Ward need to think about that (imho).