So funny story, I misread and started off playing the third Stalker game, not the first. Apparently Call of Pripyat (the one I've been playing) is last, with Shadow of Chernobyl and Clear Sky being the first and second, respectively. That story wasn't very funny.
Anyway, I downloaded the first one and started it. So far my impression is that the UI is pretty bad*. Also the combat system is just plain stupid. If you aren't aiming at the target, you will, of course, miss. If you are aiming at the target, you might also miss. You know attack rolls? Yea, it does those. So not only do you need aim, you also need RNG blessing. It's like the worst possible mix of FPS and RPG. Normally iron sights are more accurate than shooting from the hip with a crosshair, unless the iron sights aren't giving you the pixel-precision you need to even have a chance of hitting the target.
 Upon further research, this appear to be an issue in the prequel (Clear Sky), not SoC. It turns out my aiming problem in SoC was due to having really awful weapons, not a game mechanics problem.
Back on the subject of the UI: at vendors the items you have equipped are not separated out, so there's a chance that you'll literally sell the shirt off your back. The same when looting corpses. The world is dynamic, with an active AI, which sounds great, until this happens: You are sent to save a camp from bandits, which you do, killing all bandits in sight and getting the completion message, but on the way back to get your reward, a new pack of bandits show up and kill the camp. FAILED! Oh sure, it makes sense in a way, that you didn't quite save the camp, but you did what you were hired to do, save the camp, an action which I think we can separate from eternally guarding the camp.
But my real point was this: I started over again. After beating it. Yes, started playing a new game in Call of Pripyat after having beaten it maybe a day or two before. I missed a few things, such as the previously posted failed gassing of the bloodsuckers, but I missed something else: the gameplay. It was just plain fun. Going through the tunnel to Pripyat (strangely I never got a call, perhaps they're not being literal), even when I knew when I would be attacked, it still made me jump.
It wasn't all trivialized. Bullets were still bullets. Oh but yes, my gear was better. I mean, I had a better idea of where to get some decent weapons, so I wasn't trying to use an AK variant as a sniper rifle and I didn't waste all my weight carrying around sausages (noobs are pretty funny in their stupidity). Maybe that's part of what I liked: doing it better. I knew where I had set up barriers to my progress and I more quickly removed them.
That's a process that I enjoy: learning and putting it to use to do things better. Not memorization (though I'd be lying if I said I didn't learn some scripted events), but learning methods to make my virtual life easier and less fear-inducing. Things like knowing that I don't actually need that stupid box at the bottom of a giant hole filled with snorks, so I can wait until I have a badass exoskeleton and a slightly less awesome shotgun with a nice 10 shot magazine. Or simpler things like not carrying heavily damaged weapons because no one wants to buy them and repair can cost more than the value of the weapon.
Then there was that good old exploration thing. I missed a few holes. Turns out one of them had zombies, zombie rats, and the legendary Oasis which can cure all my ills! Also, a psychic dog that induces nightmares and hallucinations.
I cannot imagine ever leveling through a starter zone in WoW and immediately thinking, "hey, I should delete this guy and do that again!" Yet it another game that's pretty much what I did. It's strange. Can a MMO never match the gameplay of a single-player game? Certainly lag will cause some limits, but even beyond that, there seems to be something lacking. Story-telling? Certainly not graphics, which aren't bad, but when they're turned down for older hardware, lots of the spectacular lighting and weather effects are lost, so it's not blowing away WoW, which beside that has good bad graphics anyway. Maybe it's just the satisfying element of guns, and no corpse runs.
P.S. Buy Call of Pripyat, it's an awesome game. Buy Shadow of Chernobyl if for some reason you like horribly unpolished games. Oh, on that subject, I forgot to mention that sometimes the translation (it's originally Russian) is bad, as in, opposite, as in, it says the stuff is in the basement when there is no basement and it is in fact in the attic.