http://www.kvraudio.com/forum/viewtopic.php?t=392159&highlight=
Essentially it means plugins will not be able to access the deeper workings of the file-system, and they wont be able to call external processes, nor will it be able to communicate with other processes... meaning even something as simple as authorizing a plugin from your host all of a sudden becomes a MASSIVE ballache for the user and developer alike. You can't even elevate your permissions temporarily (very common during an installation) without digital signatures from Apple.
I don't know for certain, but it looks like Apple are trying to push all audiounits to go through the app store, because they want their 30% cut - at which point... goodbye 3rd party plugins for Logic!
It would be uncouth of me to make a Nazi-Party comparison right now.... I'll leave Godwin at the door.