* Facebook feature is integrated: Here’s a quick way to expand your social network by seeing who among your existing Facebook friends also has a Battle.net account.
* 3v3 and 4v4 formats are now enabled.
* Numerous updates have been made to the Leagues & Ladders system:
o Removed Copper League and added Diamond League above Platinum League.
o Player ratings start at 0, rather than 1000.
o No longer displays loading screen odds in placement or practice league matches.
o Matchmaking system logic updated.
* UDP is enabled to help improve game performance.
* Numerous performance and stability improvements.
Balance Changes
* PROTOSS
o Sentry
+ Force Fields can now be destroyed by Massive ground units walking over them.
o Void Ray
+ Range decreased from 7 to 6.
o Warp Gate
+ Subgroup selection priority changed from 2 to 3 so that it takes priority over Gateways when selected.
* TERRAN
o Marine
+ Stimpack research cost decreased from 150/150 to 100/100.
+ Combat Shield research cost decreased from 150/150 to 100/100.
* ZERG
o Infestor
+ Infested Terran spell removed.
+ Frenzy spell added:
# Costs 25 energy.
# Targets a single biological unit which deals 25% more damage and is immune to snare, stuns, and mind control for 30 seconds.
o Overseer
+ Contaminate spell added:
# Costs 75 energy.
# Targets a single enemy structure which cannot train units or research upgrades for 30 seconds.
+ Infested Terran spell added:
# Costs 125 energy.
# Infested Terrans have the same stats as those previously created by the Infestor and are placed directly under the Overseer when spawned.
o Ultralisk
+ Life decreased from 600 to 450.
+ Damage changed from 25 to 15 (+25 Armored).
+ Damage versus structures increased from 60 to 75.
Hotkey Changes (English Only):
* Zerg Set Worker Rally Point changed from R to G.
* Zerg Spore Crawler changed from W to A to avoid conflict with the Select All Warp Gates hotkey.
Battle.net Interface
* Revamped summary pages for player Profiles and Leagues & Ladders.
* Added a Help system with tech trees and other tips and tricks.
* Removed identifier from the character naming process and added the ability to refer friends for invitation into your party or lobby.
* Updated the Battle.net user interface to consistently use a nested menu system.
* Added in-game blocking and player muting.
Here's what Blizzard has to say about these changes in the accompanying "Patch 13 Situation Report":
In this patch we are looking to address several balance issues that the community has discovered over the last few weeks. We are also addressing some of the zerg units and abilities that don’t see a lot of use in competitive games. These changes are big enough that we think it’s likely there will be further balance changes to these units as we learn more about how they are playing in the game.
Terran

Terran Upgrades - Marine - Stimpack research cost decreased from 150/150 to 100/100. Combat Shield research cost decreased from 150/150 to 100/100.
As always, we are continuing to watch the data as we make these changes as well as are continuing to watch for constructive feedback on these changes and more.
Protoss

Sentry Force Fields - Force Fields can now be destroyed by Massive ground units walking over them.
Void Rays - Range decreased from 7 to 6.

Ultralisk - Life decreased from 600 to 450. Damage changed from 25 to 15 (+25 Armored). Damage versus structures increased from 60 to 75.
Infestor - Infested Terrans spell removed. Frenzy spell added: Costs 25 energy - Targets a single biological unit which deals 25% more damage and is immune to snare, stuns, and mind control for 30 seconds.
Overseer - Contaminate spell added: Costs 75 energy - Targets a single enemy structure which cannot train units or research upgrades for 30 seconds. Infested Terran spell added: Costs 125 energy - Infested Terrans have the same stats as those previously created by the Infestor and are placed directly under the Overseer when spawned.
Links
Battle.net - Patch 13 Notes
Battle.net - Patch 13 Situation Report