COD4 Multiplayer is really great. You gain XP by playing, and as you "level up" you earn new weapons and achievements. Achievements allow you to get more attachments for each of your guns. (Ex. New camo or scope) I think once you play you'll get addicted.
My honest opinion on the game: I love it, especially multiplayer. The unlockables just added to the experience. There are like 5? (this if off the top of my head) game modes as well, and each of them are pretty unique and fun in their own way. My only problem with this game is that I wish there was a few larger maps for sniping or more players. I say this because some maps don't play well with 32+ players... they're just too small.
It is fun but lacks depth IMO. It is good if you crave a fast paced gun fest. My fav is if you can get into a good sniper game. The sniper rifles are pretty sweet along with the camo gear you get. The maps seem kind of small though when you get 30+ people in there.
I know it is pretty dead, but I still prefer BF2142 when it comes down to a good multi player FPS with some structure and larger maps. L4D is growing on me but there aren't too many maps to choose from.
COD4 is a bit more solo style, shoot and spawn to rack up points sort of game. There are team work style multiplayer modes but still mostly ends up every man for himself. Good entertainment though.
COD1 had the best maps, hands down. Powcamp, ship, pavlov, and so much more were epic but taken out in COD2, and of course COD4 is the black sheep of the family. COD5 maps are a little too big, and the bolt rifles are WAY to underpowered.