Varied, deep, with interesting combat and a nice mix of action, stealth, RPG and adventure elements. Puts Doom 3 to shame and could even show Valve a thing or two. The three areas to escape are sufficiently different that you never feel like things are being repetitive.
Solid all around, including the music. Voice acting is spectacular, as long as you remember that Vin Diesel is always Vin Diesel.
A gritty world that could be more colorful, but is rendered in exquisite detail. Arguably as good as Doom 3 or Half-Life 2, though without the former's per-pixel shadows or the latter's wide open spaces.
Like Half-Life 2, it manages to give the feeling that there's a whole world and backstory here, without trying to shove it in your face.
The sweetest unlockable there is - and it's only on PC.
Likely a legacy of Xbox code, but I was stuck for a day because Riddick wouldn't stop running backwards. Turns out the game thought my joystick's throttle (set to minimum) was a D-pad. Yes, this is the most serious error I can think up. No, I don't care that there isn't any multiplayer.