S.T.A.L.K.E.R. minus the RPG elements and with more emphasis on action.
One of the things that set it apart from the STLAKER games is linearity. It's not an open-world game. When you're done with a section, you won't come back later on so if you don't get all the loot right away in a given level, it's lost.
Also you don't have to worry about inventory space or weight restrictions when picking up weapons or ammo. But you can only carry one weapon of each type (pneumatic/shot-guns, SMG/auto-rifles, revolvers) and, it seems, endless ammo. That is, if you can find enough because they're scarce, depending on which difficulty level you play. Even at normal difficulty, you won't want to waste ammo when unnecessary, although anytime you'll be confronted with a mutants, you''ll rarely have the chance to escape without putting up a fight. Still, it's possible to buy ammo whenever you're at a station if you have enough ''money'' to spend that is.
Wether it's ammo or ''money'' (which are immaculated 5.45mm pre-apocalypse rounds used as currency and can also be used as normal, more powerful ammo with automatic rifles and smg's) you crave, you'll need to search all nooks and crannies to make sure you gather enough of those to survive your quest. Exploration of your surrounding is the key to accomplishing that. For example, if you follow a few guys, that doesn't mean you can't explore a corridor you see on your way or, if you pass by a car, why not look inside?
You'll often be rewarded with some good stuff.
The shooting mechanism is much more related to an action game like Crysis than that of an RPG game like STALKER or Fallout 3, as all weapons feel relatively more powerful (save for the shot-guns) and accurate and there's no degradation as you use them.
One of the gripes I have against the game though is the mutants that seems to be endlessly coming at you at some points in the game. That combined with scarce ammo and underpowered shot-guns (which also reload at a snails pace) can sometime lead to untimely deaths. It felt that way on a particular level and I found myself having to use the puny knife against lumbering mutants because I could not find enough ammo. I was playing Normal diffulty level at that time so I can't imagine what Hard settings would be.
The atmosphere of the game, audio and graphics are really top notch! Make sure you have the proper hardware to run the game. I have a GTX 260 videocard, 12 Gb of RAM and i7-920 processor. The game runs fine at High settings without FSAA turned on, but not as good as I would like it to.
All in all, if you liked the post-apocalyptic setting of the STALKER games but hoped to play a more action-oriented game, Metro 2033 is for you.