For those who played the original S.T.A.L.K.E.R. the interface for Clear Sky will be instantly familiar, though newcomers may find it more than a little intimidating.
You've got your map, inventory system and a hundred little things to keep track of and micromanage. You have to make sure you are well-fed, that you aren't dying from radiation poisoning, that you are using the correct type of ammo and that you aren't carrying too much weight. You have to watch your stamina, monitor and bandage your wounds to make sure you don't bleed to death.
Then, as if all that wasn't enough, a whole extra dimension has now been added into the gameplay - weapons and armour can now be upgraded instead of just deteriorating over time. This adds a whole new side to inventory management as you have to start balancing the upgrade cost and potential of a weapon against the cost of something new.
The new upgrade system ties in closely to the faction system too and you'll only be able to get upgrades and modifications from specific traders and there's only one for each faction - something you'll want to bear in mind before you poke the business end of an AK up the backside of a potential rival.
Repairing your weapons and armour is also necessary in Clear Sky and your inventory will definitely get worn down over time - a worn weapon will jam more easily and you'll be reloading excessively. That's a serious problem when you're in the middle of one of the many firefights. That said, while all repair shops can fix everything there's no apparent price differences between merchants, so seeking out different people in different factions for a quote on your rifle is pointless and disappointing. You'd hope a feature like this would take into account your standing with a specific faction, but as it is there's no monetary incentive to be nice anyone in The Zone at all.
That said, while the traders and mechanics may not treat the player any differently, nearly everyone else in a faction will and the map system has been mercifully upgraded to deal with the different attitudes of the people you'll come across.
Especially interesting is how exactly this map system works, because it isn't as omniscient as you might expect a radar system in a computer game to be and the map will not just automatically update with details of everyone around you regardless of where or who they are. Instead, the system is more true to life and the stalkers will all work together to keep the map up to date - friendly and neutral parties will update your map with the location of any hostiles they see - which isn't necessarily going to be everyone on the map. You'll still have to spot the loners on your own, though they'll disappear from the radar if you don't keep track of them.
This map system may sound simple, or maybe even slightly flawed, but in-game it actually works beautifully. There's a constant feeling of working alongside your fellow stalkers and while you're given enough information to avoid the large enemy parties there's always the chance that you'll be surprised by lone enemies wandering around. It's the best of both worlds.
What's even better is that this system seems to be two-way too and just because you get spotted by one enemy doesn't always mean that they'll call down all their allies on your position - though that is likely if there is a group of hostiles nearby. You'll have to be especially careful of enemies call for help at night too when people seem to naturally bunch together around the campfires, though you can always just cheat and turn up the gamma instead.
The enemy AI isn't always this clever though and many baddies will appear to look through walls and try to shoot through them, or stand in full sight while they reload instead of crouching or hiding behind a nearby car. In fact, if you're squaring off against a big group of enemies in the same area you can often watch the AI scripts executing the same actions for multiple actors at the same time. There's something amusing about watching ten identi-kit enemies reloading and shooting at you in unison, especially when they won't dodge a grenade or run into each other trying.
Combat itself also tends to be a little unfair too as the AI has often impeccable aim, even with grenades, and can take at least two direct headshots on average. It has to be said that the bullet physics and sheer gunplay of Clear Sky is just as phenomenal as it was in S.T.A.L.K.E.R., it's just the enemies themselves that make the combat a little shallow. It's tough to get scared of a bandit who will try and shoot at you from range with a shotgun - especially if you're equipped with a scoped assault rifle. In that respect, it actually makes the earlier gun fights with simpler weapons more fun than the later ones.