Official statement: http://forum.mql4.com/63391
It's really sad...some old EAs may not work. Build 600+ has a limit of running maximum 32 terminals...don't know what's the benefit for brokers, traders, and Metaquotes. You can remain using build 509 if you're broker also remain using the current version of MT4 server (build 600+ server is compatible with build 509 terminals now) and turn their auto update off.
It's really sad...some old EAs may not work. Build 600+ has a limit of running maximum 32 terminals...don't know what's the benefit for brokers, traders, and Metaquotes. You can remain using build 509 if you're broker also remain using the current version of MT4 server (build 600+ server is compatible with build 509 terminals now) and turn their auto update off.