Despite what demi says, CoD4 is pretty great. Great visuals, great maps (all 3 of them are great and different. You get claustrophobia and open areas), solid controls (you can tweak sensitivity among 4 levels, although I didn't find a sweet spot, it worked fine enough). There are lots of guns, and due to the great create-a-class and tweaks systems, they've done some balancing to make sure that there really isn't one uber machine gun or rifle or whatever. Most guns have advantages and disadvantages. The M16 is super powerful, but you can only use burst fire. The M4 is a great automatic, but it's not very accurate outside of close range, making it suitable for claustrophobic maps. There are also a wide range (well, wide for an FPS) of accessories for weapons, ranging from scopes to grenade launchers. Want a laser scope instead of iron sights? You can do that (but you usually have to exchange a perk/tweak slot).
Perks (or tweaks or whatever they are called) also change up the fps formula. There's of course the famous Last Stand, which allows you to pull out your sidearm as you fall to the ground dead, so you can pull off some vengeance on the fucker that just took you out, and maybe some of his friends. There are also a range of standard health and accuracy improvements, improving the ability to hold your breath as a sniper, etc. There are lots of different tweaks. . .I haven't unlocked a whole lot of them.
Unlocking stuff works on the Battlefield model. You gain points from doing shit in matches (including getting bonus points just for completing matches, rewarding peeps who stick around to the end even if their team is losing), and once you rank up, you get a new unlock, like a new weapon, perk, accessory, challenge, option, whatever.
Challenges are kind of like in-game achievements. You get some experience points towards rank up for completing challenges. A typical challenge would be to get 25 kills with a certain kind of machine gun, or 100 headshots with a sniper rifle. Typical achievement stuff. Busting out the challenges makes you level pretty fast.
The gameplay is pretty solid, as is. If there weren't only 3 maps, I'd think this could be a final product. The only significant bug (and it's not really that major) involves getting stuck while strafing on stairs, and it's limited to one map. Oh, also, when you get killed, you are treated to a pretty sweet kill cam, showing you exactly where the fucker that pwned you was hiding. I like this feature a lot, because although I love fpses, I suck ass at them, and I hate getting killed by someone and then not knowing where the dude was hiding. That shit is aggravating.
Custom classes are pretty sweet, but as of the beta, it seems like they don't get their own challenges. It kind of makes sense when you play the game (challenges are tied to specific classes), but it kind of doesn't encourage you to play with custom classes until you have a ton of shit unlocked. I guess that makes sense, too. Still, even at early levels, it is super sweet to bust out with your own modified super soldier, armed with the guns ad perks you picked.
Overall, the game is looking fucking excellent. Halo had better bring a lot more than Halo 2 did if it wants me to spend any multiplayer time on it when this is the prime MP competition this winter.
Also, the game looks amazing. Amazing particle effects. Amazing animation. LOTS of death animations. Only weak spot with animation is that the Last Stand animation kind of gives away you are doing the ability, giving enemies the heads up to bail or destroy your body.
A+++++++++++++++++++++++++++++++++