What it is now?
KB3035583 raised a lot of talking about Microsoft's way pushing their users towards Windows 10 - It is the update creating a small Windows icon in the taskbar where users can "reserve their free copy of Windows 10", available to most Windows 7, 8 and 8.1 systems. But, under some circumstances the Update isn't available to Windows users in Corporate environments even though the system would fulfill the reservation requirements. As far as I am concerned there are two cases most sysadmins will encounter:
While KB3035583 is installed on systems joined to a domain, the taskbar icon will not be displayed. Microsoft is targeting the usual home user with their campaign, and allowing users to change the OS without giving the sysadmins a chance of tailgating that is not the best idea, thus the business users can't update the OS themselves (which makes some sense).
Windows Update Services isn't fed with the KB by Microsoft - For the same reasons as the Taskbar Icon isn't shown in systems which are joined to a domain.
Windows 10 needs to be deployed using the OS deployment processes already in place, which isn't a huge drawback given the fact that most companies will already have an OS deployment process in place. Plus it gives sysadmins a chance of testing the OS image before rolling it out to their users.