Out numbered and not enough ammo
The graphics are stunningly detailed, and each weapon has distinct characteristics. Even on low settings, everything is highly detailed and it makes you feel as if you are living in the post-apocalyptic world the designers drew. Firefights are fun, and this game really tests your ability to strategize and plan your survival; whether it's your gas-masked cracked, worn filter, overheated gun, radiation, or just some damned monster, it feels like its you against the world and there is something bound to kill at every corner. The world is grim, but action packed.
As for the gripes:
First and foremost are the demanding system requirements. No matter what I did with the stock settings, there was a 30FPS ceiling I was hitting, and certain parts of the game would bring my system to a crawl - I have yet to throw a game at my machine to do this and this game did it at LOW settings. Core i5 2500k @ 4.6GHz, 8GB DDR3-1866, Radeon 5830. At first, it was a playable, but annoying 25-30FPS steady on Normal settings 1920x1080. But after some in-depth tweaking of configuration files, and some setting over-rides with CCC I finally managed to pull of a consistent 50FPS on the low end, with spikes between 70-110FPS, at which are ultimately normal settings. Didn't bother with high, because in shooters, fps takes priority over eye candy. Going from 25 to 70 FPS says a lot about this game: it's not overwhelming graphics, but bad coding that makes the demand from hardware.
The difficulty is the other gripe. I haven't read about many people sharing my opinion, so maybe I'm just rusty, or everybody is playing on easy, but this game is entirely too hard, almost to the point where it stops becoming enjoyable. I was dying on the first level after the prologue... multiple times!
Ammo is entirely too scarce. I fancy myself a marksman, fairly accurate, and usually bursting automatic weapons if no single-shot option is available. Even then, I found myself out of ammo entirely too often. 7 rounds in one monster to kill em from an SMG, and you only pick up about 3-10 rounds per (human) body. And combine that with large swarms of enemies, and a knife that's no more effective than a cotton-ball... well you get the picture. And on top of the scarce ammo, most of the weapons (at least in the beginning) have low-capacities. Even cycling through all my weapons (pistol, shotgun, AR/SMG), I still have to reload multiple times in a single firefight. And you won't believe the best part; I eventually modded the config files for infinite ammo, and I was still dying from the low-capacity mags from the early weapons and the large swarms.
Maybe I'm just rusty, but this game does seem to be entirely too hard. I've never had a problem with ammo in any other game (I'm an ammo hoarder), whether it be Crysis, Halo, Modern Warfare, Battlefield, or whatever, I usually die with a LOT of ammo. IMO, the hard difficulty is for the ultimate FPS junkies, while normal is for the gaming veterans who want a challenge, which I would hardly call normal.
HOWEVER, the difficulty doesn't kill the game. I'm still making progress, though significantly slower than what I'm use to. The ultimate verdict here is fun, but awfully challenging. And the high details makes the play-through pretty enjoyable, not matter how many times you have to replay a checkpoint because you died.