updated 01:54 pm EDT, Tue September 2, 2014
Changes so far undiscovered
[Updated with some early changes] Apple is now distributing a seventh developer preview of OS X 10.10 Yosemite, as well as matching betas of Xcode 6, OS X Server 4.0, and Configurator 1.6. Any changes to Yosemite are unmentioned in release notes, but will probably be uncovered within the next few days. The last preview build of Yosemite was released roughly two weeks ago.
Officially, the finished version of Yosemite should ship sometime this fall. Reports have typically pointed to October.
Update: Early anecdotes indicate that changes are mostly cosmetic, for instance swapping out old icons for new. One important tweak is the removal of "Software Update" from the Apple menu. Instead, users are presented with an "App Store" option, accompanied by a badge showing how many apps need updating.