I followed this game since 2003, been on their forum since 2004. Hot damn they delayed that game so many times

, thankfully they slipped us a few alpha and beta builds over bittorrent once in a while so our pain would be eased a bit.
As one of Stalker's loyal early fans, I have to say the early concept of this game was phenomenal, revolutionary even. But the devteam was very understaffed, and underpaid as well I heard, causing them to delay the game AGAIN and AGAIN and AGAIN (definitely over 6 times, for a total duration of 4 years), culminating in the take-over of THQ (the publisher) of the Stalker staff. They sent some American IDIOT (called Dean Sharpe) down there to show those Ukrainians how to make a good game. Needless to say, they dumbed that game down hard, took out all the interesting and innovative features in favour of keeping the game simple and easily accessible to North American audiences (IMO this is the cancer that is causing the stagnation of the gaming industry right about now).
Scrapped game features, they're terrible and numerous. Originally the game would be based around a system where all monsters, people and anomalies would migrate throughout the gaming world, called A-life. They scrapped it. It may look like the world is migrating in Stalker but in reality it's all scripted. It's actually possible to reactivate the disabled A-life system through modding, bringing the whole world back to life. Someone may give you a quest to take out some dude who lives near a village, but when you actually got to the village the dude had gone artifact hunting in some dangerous area, and the problem would sometimes take care of itself. It can mess up the game storyline ofcourse and takes some finetuning, because the characters giving you the missions walk off all the time.
The game would originally contain periodic blowouts that reshaped the whole Zone and move the anomalies around. Blowouts were also dangerous and lethal, and would have been a major factor in keeping the game envirnonment varied by killing off all the monsters and careless stalkers, so the void would be filled by immigration from other regions of the zone, upsetting the balance of power. They scrapped that as well. The only occurring blowout in the game is scripted and happens at the end of the storyline, and doesn't do anything to the environment of the Zone. It's just a bunch of flashing lights and a loud boom noice, that kills you.
Drivable vehicles, customizable weapons and sleep were also scrapped. It's also nigh impossible to join one of the ingame factions, although using mods you can align yourself with anyone of them (picking the Thieves' side and walking around in one of their uniforms stealing money from rookie stalkers is great - although you can't get sidequests now). It's a shame they didn't retain this posibility in the vanilla game. You also can't influence the apparent stalemated conflict between the factions (as the devteam originally stated you could), unless you turn on that A-life system. Thankfully they'll fix that part in Clear Sky.
Also, they made a few right fuck-ups with the storyline. originally the most interesting and often celebrated visitable area in the game would have been the surroundings of the powerplant, and Pripyat (the city). You can visit them in the game, but only on one single occasion during a story mission, right before the game ends. When you visit the reactor, you can't even go back anymore! You're stuck there until you finish the game. And when you complete the game you can't keep playing the game with a full inventory, do some random missions and explore the remaining side quests ETC, when it's over it's over and you have to start anew from scratch. That left me with a very disappointed feeling.
Also, the multiplayer is amazing and extremely tactical, a bit like playing Gears of War or Counterstrike. But it HAS to be played by LAN, because the netcode in this game completely break the multiplayer. WAY too much lag. Hackers all over the place. It sucks, because the multiplayer has some amazing potential.
I hope they'll fix some of it by the time the new expansion comes out, so I can play the game I waited 4 years long for.