Was reading a very interesting thread and it was nice to see everyone's thoughts and feelings on co-op in any game not just Stalker. The most annoying thing about stalker is the "game-play" is so well suited to co-op. Like with all the animals walking around, the environment changing (both with weather, time and AI events, every zone changes all the time and your actions do make a difference which is great, since the world lives not only around you but you are an important part of it). Just the idea of being able to experience this living world with a friend is very exciting.
Bandwidth could be an issue as it is so very important in a first person shooter. The problem is for a first person shooter, everything tracked on the server has to be tracked damn fast since everything moves fast. When you shoot a bullet, it must check hit against the object you were aiming at in real-time and if the client and server is slightly out of sync even though you really are sure you got your aim right, you'll keep missing (which is what you'll get with a bad ping)
Ok, now take an MMORPG like World of Warcraft, you click on an enemy, select your attack and watch it play out, each hit or miss is calculated and then fed back to the client side which means if even if your lag is really bad, you won't notice because the event is being run and fed back to you in it's own good time.
Take COD4 and do a simple little test. Get a can on the ground and shoot it, now get your friend to look at the area where you are pointing and ask him if he can see the can... ok, I won't make you do it, but trust me he can't (I tried) because the physics calculations and position of all those tiny objects that go flying around the level is all done on the client side, why? Because you would waste a lot of bandwidth sending all those numbers around the network for the position of each tiny objects and it would have no impact on the game whatsoever! The calculation is done on the each individual client yes, but to track every object on every client and make sure it is the same for each client would seriously kill the server.
Now take Stalker, each animal is running a pretty complex AI, not only in the zone your in, but the A-Life in each zone is tracked (so when you move between zones, things are similar to the way you left it). Even if that AI calculation is done on the client side, the positioning for each AI has to be tracked, sent to the server and then fed to each client and you must ensure that each client has the same set of co-ordinates for everything single object in a very large zone. That's a killer to keep track of and code (trust me, I'm a games programmer)
Apart from that, take into account that Stalker was in development for a very long time, over 7 years I think. It changed hands a few times and even the single player campaign is very buggy. To get anything as complex as A-Life to work on a server, you'd have to plan it from the beginning of the project and would need some serious tech!! possible, maybe but feasible for the amount of people that would actually play the game JUST for the co-op, most likely not.
Still, the game rocks, so I hope they figure it out!!!