the DHCP problem fixed (there might have been more, but I have only read about one fix to it)
was, in 8.5 (& 8.5.1) it would send a DHCP RELEASE when the machine was shutdown/restarted, then when it started up again, it would assume it still had the same IP address from DHCP, and just ask for it again, of course, it was released, so this would cause errors (and a long delay in start-up)
in 8.6 it now does not release (there is still an option to force it to release)
problems with DHCP servers on seperate subnets is a completely different problem, and I'm pretty sure MacOS isn't the only system that would have a problem with that =)