Beyond the complete game drops that seem to be the result of a the game servers at Blizzard crashing, there still seems to be a high amount of in-game lag (slow unit response / minor freezing / random drop screens / etc) in Patch 13. I suspected this was the result of poorly implemented UDP networking code, so I wrote a program to test this theory.
By default, if SC2 detects you are able to send and receive UDP packets, it will default to the new UDP netcode in Patch 13. If your UDP is not usable, it reverts to TCP as in previous versions. By blocking UDP, you can force Starcraft II to use the old TCP code. After very early testing in a few 4v4s, this seems to have helped reduce the amount of in-game lag.
EDIT: Note that this won't magically fix all lag instantly, as other players in the game may still be using UDP. See this for more information. At best it prevents you from causing lag or dropping.
Use at your own risk, etc. Requires Visual C++ 2008 runtimes. Please post feedback whether this helps or not, it will be useful to know if this is actually the reason as I am not able to do enough testing by myself.