I just upgraded one of my desktops from Wheezy to Jessie, and had a few issues. For the first, I can share the solution, and for the second, I have questions, and perhaps complaints.
The first issue was the video -- the machine has "legacy" Nvidia cards in it, and I've been using the proprietary NVidia blobs for years, straight from the NVidia site. This file didn't work for Jessie, it couldn't find some header files -- I wasn't too concerned, I figured at this point that old NVidia card (it's a GTX 260) is probably supported by the Nouveau drivers. But, the Nvidia uninstaller won't work either, and the nvidia-installer-cleanup package failed its config. Turned out what it was doing was trying to run /usr/bin/nvidia-uninstaller, which I think is the same NVidia-provided executable that doesn't work. The solution is to just remove that executable, and then reconfigure/reinstall the nvidia-installer-cleanup. The last issue was, I was stuck in low-res VGA mode until I remembered to go clean out the nouveau blacklists from /etc/modprobe.d. It was blacklisted three or four different ways. And, I was right, the nouveau drivers work just fine for this card.
My second issue, still unresolved, is with the new version of the grub bootloader, grub-pc 2.0.2. It defaults to "restricted" mode, so that as-installed, the system requires the root password to be typed in at the console before it will boot. For my secondary desktop system at home, this is merely inconvenient, but for headless servers in far-away data centers, this is potentially extremely serious. I surmise from the bug reports (eg. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=708181
) that the grub upstream wants this behavior, but really, it needs to be dead simple to get the machine into a state where, absent errors, it will boot autonomously. At the moment, the solution in the Jessie release seems to be to either remove the superuser entry (opening up the whole grub process), or hacking a "--unrestricted" flag into /etc/grub.d/10_linux, and hoping it doesn't get clobbered by a package update.
It's possible there's an easy workaround that I just haven't found yet, but this is highly undesirable behavior for the server use-case.
On the up side, so far, Jessie seems quite stable, KDE worked fine, and "so far so good" on systemd.