Graphic/sound improvements:

New tree models / textures (Absolute Nature). Each level was edited to add more and new foliage.
New weather system (special edition of AtmosFear for Autumn Aurora 2).
Lots of new sounds of environment, weapons, items as well as level-specific sounds and actor voice.
Environment sounds are dependent on weather type and time of day. Moreover, each weather type boasts totally different and unique sounds.
Lots of new edited textures.
New music.
Gameplay:

Some new weapons (with unique NPC's and as rewards for quests) and items (not many, maybe 10).
Artifacts detectable only by means of detectors (Echo, Bear and Veles. Each detector has distinct characteristics). Read more about it here: Moddb.com
Two modes of repair (traders as well as outfit and weapon repair kits).
Sleeping possible in designated areas (like in CoP).
New UI/HUD (CoP-based).
Hand texture adjusts to the suit you're wearing (like in CoP).
HUD and breating sounds are mask-dependent.
Random stashes.
New screen effects when irradiated, on low health and out of breath (low stamina).
Applying medkits, eating or using repair kits takes time.
New recoil effects.
New nightvision devices.
New postprocess effects for anomalies, radiation etc.
Some artifact models are CoP-based, and some are new.
Stalkers will carry Russian weapons for the most part. U.S. weapons like LR300 or M16 have been assigned to one faction only and they will be almost a rarity. If you happen to own LR300 or M16, you will have a serious advantage over your adversaries. This applies to exoskeletons or SEVA suits as well - they are finally very exotic and expensive items, but will definetely be worth their price.
The game itself will be more difficult, but well-balanced to avoid being unfair.
Some bugfixes like empty stashes and non-disappearing mission markers.