STALKER - PreSky changes many features from vanilla (un modded) Clear Sky. Below is a list of features you can expect to see in the first version
Stalkers and monsters alife interactions have been changed for added realism and restoring some of the interaction between them that was present within STALKER - Shadow of Chernobyl.
Stalkers: Stalkers abilities are now depending on faction. Panic thresholds, accuracy, listening, sight, etc are now determined by the faction the npc comes from. They will now limp if badly wounded enough, can also bleed, and they now recover health slowly like the player. They will also use medical supplies in their inventory if they have them and are injured enough.
Sight parameters have been adjusted, so stalkers can no longer see you through walls or through bushes in the middle of the night. Listening parameters have been adjusted, so stealth is slightly more effective now.
Mutants: Mutants have undergone various adjustments so they act like their description indicates, and have properties based on their official descriptions. E.G. Blind Dogs now act blind, and some mutants cannot see as well during the night as during the day. Mutants also have a tendency to migrate towards certain smart terrains during the day.
Zombified: Have had a few of their parameters fixed, so they are now more like what they should be.
Free roaming Stalkers and Mutants are being introduced as well. They will react to any threat within range, such as hostile Enemies or mutants, either free roaming or smart terrain based. These guys can be a bit touchy sometimes, so be careful.
With the listed alife features above, along with new and restored Weapons, armor, and other items, coupled with new quests and redesigned upgrade schemes similar to those in Call of Pripyat make for a different gameplay experience. The Faction War might not play as bigger part, but there are still rewards to be had from it, and various skirmishes still occur on the overlapping edges of various factions' territory.
New Content: With several new weapons, all of which are used by npcs, and new suits available to the player, along with several new npc skins, gives a more diverse and varied appearance of stalkers.
Restored Content: Several cut mutant types, along with cut and unimplemented/unfinished ammo types have been restored, to further enhance the zone experience.
Npc Spawns and Appearances: New npc spawns have been added to take advantage of new weapons and skins, and pre-existing spawns have been redone to reflect the equivalent suit, weapon, upgrades, and gear that particular npc could afford or could have scavenged. Also, npcs can be loosely grouped into classes dependent on weaponry and equipment loadout
Stashes: You can still find Clear Sky's regular stashes by looting corpses, but this mod also features open stashes, which as the name implies, are items lying around in the open. These are not marked, and are often hidden, requiring the player to search for them. They may contain important items, weapons, ammo, suits, or medical supplies for the player. Some even contain rare artifacts.
Weaponry: Weapons and ammo have undergone a slight change to improve realism. Rate of fire, bullet speed, fire distance, and weight have all been more or less brought back to their real life equivalents. Although still not accurate, there is less watching bullets sail harmlessly past an enemies head.
Improved Looting: It is now possible to loot virtually an npc's entire inventory upon death, ie, what ever they had for trade, you will be able to loot, along with any items they might have picked up along the way.