You do realise that we're still three months away from the "real" release of Win 10? The Threshold project has for ages been geared to an October 2015 GA. Some time early in the new year Nadella decided they've got to shoot for Back to School (a huge seasonal driver of new PC sales in USA), and Threshold was split into a two-step release, one tomorrow and the "missing bits" in October 2015. This is possible because of the new WaaS model, which is just another way of saying "ongoing updates, new features & functions, etc released as we finish 'em".
Insiders remain part of the testing and debugging process for future enhancements, and in the latter part of August we should start seeing some Threshold II bits.
This staged release is nice for us. We get a pretty stable release now (10240) some months before originally planned, so an early leg up into the new platform.