This game goes haRd...
I'll briefly echo all the other compliments here: nearly photo-realistic graphics; pinpoint control, targeting, and collision detection; silky smooth operation in XP. Voice acting is believable. Weapon and explosion sounds are appropriately metallic and jarring. And the undead are scary as hell. I've played many of the good survival horror franchises, and none except Silent Hill 1 freaked me out like the zombies in Wolfenstein. It was a rare stroke of genius to introduce the Nazi grunts as sympathic victims of the undead hordes, showing scenes of your enemy's enemies devouring them alive. This creates a real hesitation and apprehension about the player's role that exacerbates the fear. In other words: me scared, good job.
So, if it rocks so hard, why not five stars? Well, this little revelation hit me about halfway through the game: instead of being the best Castle Wolfenstein it could be, id decided to make a very good cross between Medal of Honor and Wolfenstein. Which is a very good thing, but remember what set the first Wolfensteins apart from the rest were the exploration and "secret area" goodies. RtW would have been great if every level had been more like Paderborn (the best stage of RtW): more dungeon crawling, deeper and more inventive secret areas. I'm imagining innovative and useable secret goodies that enhanced gameplay, hidden by new concepts of concealing secret doors. You know, play to your strengths rather than copy someone else.
As it is, Return to Wolfenstein is a fun, pretty, and often scary offbeat shooter that's just a hair below the best of the more traditional WWII FPS.