Via [url=http://blog.counter-strike.net/index.php/2015/09/12496/]the CS:GO blog[/url]:

[quote=Valve][ ANIMATION / GAMEPLAY ]
– Replaced all player body animations (Existing character models retained for demo compatibility)
– Replaced all world model weapon animation
– Updated shared player skeleton
– Re-rigged all player model geometry and player scaffold animation
– Updated animation networking to continually synchronize animation state instead of periodically latching
– Player animation sequence selection is now server-initiated
– Added new player states including bomb defusal and ladder climb poses
– Added thirdperson weapon deploy animations to more accurately represent viewmodel deploy state
– Extended all vertical aim matrix ranges to encompass the majority of vertical aim pitch so weapons line up precisely to firing angle
– Added reverse-bone-merge animation support so weapon models can store and play independent animation on the player holding them
– Added support for arbitrary numbers of articulated mechanical parts on world weapon models
– Updated weapon world model articulation points, bodygroup presets, holster positions and reverse-bone-merge skeletons
– Added physics motion to holstered attachment weapon locations
– Added support for non-polygonal capsule-shaped hit volumes, defined by a line segment and radius
– Replaced shared hitboxes with new capsule-based set
– Rebuilt player ragdoll angular constraints, physics hulls, and interaction pairs
– Ragdolls now assume more exact pose of their parent player on physics init
– Re-built animation statemachine to support blending locomotion over any weapon aim or action poses
– Enabled support for dynamic player animation layer re-ordering
– Sequence blendlayers now correctly contribute to computed cyclerate
– Added defuser cables and multimeter model
– Added hlmv support for capsule rendering, submodel attachment previewing, ragdoll constraint limit and bone name rendering, plus numerous small fixes
– Added model format support for named bodygroups, combined physics hulls, activity modifiers and animation system specific metadata
[ UI ]
– Fixed a bug where the mini-scoreboard armor/helmet information was not displaying for coaches
– Added sv_show_voip_indicator_for_enemies server convar. If set, it will make it so the ‘voice chat’ icon shows over enemy players as well as allies when they are using their microphone.
– Added sv_showbullethits feature to visualize server-confirmed hits and near-misses on listen servers
[ RENDERING ]
– Fixed improper stencil state in glow pass rendering
[ GAMEPLAY ]
– Added dynamic player culling to prevent players and weapons poking through thin walls and revealing their position
– M4A1-S:
— Reduced price
— Reduced armor penetration
— Reduced ROF
— Increased base spread
– Zeus x27
— Reduced price to $100
– Dual Berettas
— Increased armor penetration
— Increased range modifier
[ GOTV ]
– Added viewmodel position lerp to gotv camera transitions
[ OVERWATCH ]
– Players can now report enemies for anti-competitive griefing (i.e. deliberately losing).[/quote]

Rumor has it:

- [B]Some matches may run into delays tonight[/B]
- [B]If you or your league do NOT want to play with the balance changes applied[/B]: a previous version branch under the name of "sep_03" is available, which will be the state of the game as of [url=https://www.reddit.com/r/GlobalOffensive/comments/3jjtso/counterstrike_global_offensive_update_for_9315/]the 9/3/15 update[/url]
- To opt in on the client side: go to CS:GO properties in your Steam library by right-clicking on it and clicking "Properties", then go to the "BETAS" tab and select "sep_03" from the dropdown, and Steam will reacquire files from the time of that update
- To opt in for servers: "+app_update 740 -beta sep_03" in SteamCMD

- VPK contents diff (lots of stuff changed!) and strings: https://github.com/SteamDatabase/GameTracking/commit/064363c06b491431c89ef841678907769f69c99b

- C4 script change: https://github.com/SteamDatabase/GameTracking/commit/cdb11985c0bcb6268dbd0d147ce502640cfe24f6

- Size is ~100 MB