The Steam News site has the details of the much vaunted TF2 heavy update, as well as the changed to the heavy class, a new game mode and two new maps are also added.

Added 3 Heavy unlockables

* Natascha, The Sandvich, The Killing Gloves of Boxing
* Added 35 new Heavy achievements

Added Arena game mode

* 5 arenas: Lumberyard, Ravine, Well, Granary, Badlands

Added 2 new maps

* Badwater Basin, a new payload map
* cp_steel, a community map by Jamie "Fishbus" Manson

Changes

* Added server map timeleft to upper right of scoreboard
* Added new firing sounds to The Backburner
* Removed the Custom Tab in the serverbrowser, and added the Tags field to the base Internet Tab
* Increased The Kritzkrieg's uber-charge rate bonus from 10% to 25%
* 32 player servers can now support an extra slot of SourceTV
* Made the freezecam item panel description easier to read
* Players now broadcast whether they chose a team directly or use autoteam
* Added proper handling of cart blocking recognition to Payload maps

Bug fixes

* Fixed bug where switching from flaregun-wielding Pyro to another class meant you didn't get a full ammo loadout on first spawn
* Fixed Spy watch arm not using team skins
* Fixed weapon switching preventing you from using secondary attacks for a 0.2s window after the primary became available
* Fixed players getting stuck in a bad animation state when class switching while spinning the minigun
* Fixed a loophole that allowed players to pickup weapons of other classes
* Fixed a bug that resulted in players earning more crit chance bonus in large, single damage events than they are supposed to
* Fixed an issue with the floating HUD +X health indicators that resulted in them appearing incorrectly
* Fixed a bug which caused the minigun spinning & firing sounds to occasionally stop while the Heavy was still firing
* Fixes the Loadout player model panel causing the in-game player facial animation to perform incorrectly
* Fixed spectators not having the target ID labels display correctly
* Fixed the cart alarm sound occasionally getting stuck on in Payload maps