Updated March 2nd 09: read my full review of Call of Duty: WaW here

Sum up:
Call of Duty: World at War is said to be the last WWII-set game in the series, and takes place at the tail end of that war. This game will include more interactive and destructable environments, and you'll able to shoot large holes in walls, destory areas with your flamethrower, etc; it features less linear levels, with more paths to choose from; Gary Oldman and Kiefer Sutherland voice two of the characters; and it's touted to have an even more mature tone than the last installment.

Recommended age:
The ESRB rated this game "M" for Mature, listing "Blood and Gore," "Intense Violence" and "Strong Language." The "Blood and Gore" will consist of moderate amounts of blood splatter when you shoot or hit an enemy, and there's many gory scenes depicting war at its most realistic—including a scene near the beginning where you witness an ally tortured by a Japanese soldier, who is then finished off by having his throat slit with a Japanese katana (sword).

The "Intense Violence" refers to the fact that you'll be killing hundreds of enemies with a variety of weapons, along with plenty of explosions and very intense scenes involving the main characters. The "Strong Language" means there is many uses of words like d*mn, hell, bastard, b*tch, a**, sh*t, and a few uses of the f-word. However, there is a graphic content filter that removes all gore and the majority langauge, so I'd recommend this for ages 14+.