I have solved the problem for me, hope this helps others.
I have a six plus and a 5S. The 6 plus had the same problem that has been described in this thread - dropped off Wifi, then impossible to get Wifi back. Looking at logs on the router, it appeared to be a DHCP issue. I then tried all the suggestions in this thread (disabling location services, reset, etc etc.) nothing worked. However, in the mean time, my iPad air iOS8 and 5S still had a connection, so that ruled out iOS8. Then one day, the 5S lost its connection too. No matter what I tried I was unable to get both phones back on Wifi.
At this point I noticed something in the log of the router - it reported that it could not sync to the NNTP server. The default NNTP server on my router was some weird un-heard of host. I changed it to time.apple.com, reboot and voila, DHCP started behaving properly again. And all my devices are behaving as they should.
I suspect that, as also reported in another reply above, the iPhone is ignoring router ack requests because time may not be in sync between the router and phone. I have no idea what the tolerance is, but to play it safe I use Apple's time servers on all the devices now, and everything is ticking in synchrony, and back to their reliable selves.