At least one new gun. It appears to fire at least three plasma rounds in shotgun-like fashion. Note that its exact features/perfomance may be changed later, and its name is unknown.
At least one new game mode, Team vs Team. This is likely the credits based gamemode.
The game will now display the exact amount of damage dealt by each shot. This can be turned off in game settings.
Jetpacks, or at least some jetpack/booster-like equipment/items/abilities. Although it's unknown what would jetpacks look like and what exactly would they do, it's known that selfboost technique will somehow be considered a jetpack, along with double jump.
Indirect illumination & sun light effects. Ambient lighting completely removed.
Throwable grenades are returning.
AI optimizations and trajectory predicting for grenade launcher-like weapons.
AI players are now able to make ricochet shots that can hit opponents hiding behind impenetrable walls.
AI can calculate possible damage for penetration-based weapons in PB2.5.
Friendly fire and team damage factor can be configured individually for every team in levels in PB2.5.
AI in PB2.5 can perform wall-jumps.
Computer-controlled players can swim.
Al players can use jetpacks.
AI players can gather information from other allies and make blind shots on targets.
Added collision check for projectiles and weapons. Bullet damage/speed will no longer decrease on gun collision.
Dynamic lighting has been added to every bullet, grenade, explosion, muzzle flash, as well as swords. These can all illuminate dark areas. Explosion light is based on explosion color.
Player bounding box (AKA hitboxes) improvements have been added. They should let player's movement feel more based on player's body instead of the bounding box.
A "blaster" effect for projectiles.
Projectile spawn offset instead of spread for certain weapons. This means that bullets, instead of coming from the middle of the player, actually spawn at the barrel of the gun.
All projectiles have hit points now. Damage needed to destroy projectiles depends on guns: for example, the Assault Rifle C-01r needs 3 hits, and the Pistol C-01p needs 2 hits. Beam weapons do damage to rockets/plasma/grenades without knockback.
Bullet-bullet intersections. This allows bullets to interact with one another. However, the bullets are not destroyed. Other projectiles can interact with each other as well.
A Clan system has been confirmed.
AI can throw and try to dodge hand grenades.
Chest and pelvis are destroyable now. (Added in v1.28)
Blood stays on background.
Pistols can be dual-wielded. This seems to cause major balancing issues, though. Other weapons can be dual-wielded in the level editor, depending on map author's preference.
A possibility to make sloping surfaces. However, this is making calculations 30% slower, so this may not be a feature appearing in the release.
A credits-based game mode is planned as a second main mode for Multiplayer. In this mode, the players are expected to be able to buy weapons (and likely ammo) using money they have earned by killing enemies (or/and completing some special objectives). It's not clear how exactly this system would work.
Advanced pathfinding for AI, allowing them to move upwards at will or travel the map. 
AI may be able to use the "X" button just like players. 
The Falkok ship may be a usable vehicle in PB2.5. 
The first screenshot ever published for the game.
"Blood holes", or just "holes". Note that holes for robots were not implemented yet when this screenshot was taken.