600. gdm/gnome breakage in debian jessie October/November 2014

I did a dist-upgrade last night and encountered this on my next boot:

During the dist-upgrade I got a message about the nvidia driver version not being compatible with my system -- small surprise as I have an onboard intel chip and no external graphics card (it's a lenovo thinkpad SL410).

Installing kdm let me at least boot into a DM. I still couldn't launch gnome from kdm, it giving the same error message as in the photo.
Digging a bit deeper and looking at what happened before/during a crash:

journalctl

Nov 02 10:46:30 niobium kdm[1317]: Stopping K Display Manager: kdm...done.
Nov 02 10:46:30 niobium systemd[1]: gdm.service: control process exited, code=exited status=1
Nov 02 10:46:30 niobium systemd[1]: Failed to start GNOME Display Manager.
Nov 02 10:46:30 niobium systemd[1]: Dependency failed for X11 Display Manager.
Nov 02 10:46:30 niobium systemd[1]: Unit gdm.service entered failed state.
Nov 02 10:46:30 niobium kdm[1350]: Starting K Display Manager: kdm.
Nov 02 10:46:30 niobium sudo[1311]: pam_unix(sudo:session): session closed for user root
Nov 02 10:46:30 niobium console-kit-daemon[1707]: (process:1372): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 02 10:46:30 niobium console-kit-daemon[1707]: (process:1373): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 02 10:46:30 niobium console-kit-daemon[1707]: (process:1374): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 02 10:46:30 niobium kdm_greet[1378]: Cannot load /usr/share/kde4/apps/kdm/faces/.default.face: No such file or directory
Nov 02 10:46:31 niobium systemd[1]: gdm.service: control process exited, code=exited status=1
Nov 02 10:46:31 niobium systemd[1]: Failed to start GNOME Display Manager.
Nov 02 10:46:31 niobium systemd[1]: Dependency failed for X11 Display Manager.
Nov 02 10:46:31 niobium systemd[1]: Unit gdm.service entered failed state.
Nov 02 10:46:32 niobium console-kit-daemon[1707]: (process:1398): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 02 10:46:32 niobium console-kit-daemon[1707]: (process:1399): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 02 10:46:32 niobium console-kit-daemon[1707]: (process:1400): GLib-CRITICAL **: g_slice_set_config: assertion 'sys_page_size == 0' failed
Nov 02 10:46:32 niobium systemd[1]: gdm.service: control process exited, code=exited status=1
Nov 02 10:46:32 niobium systemd[1]: Failed to start GNOME Display Manager.
Nov 02 10:46:32 niobium systemd[1]: Dependency failed for X11 Display Manager.
Nov 02 10:46:32 niobium systemd[1]: Unit gdm.service entered failed state.
Nov 02 10:46:33 niobium systemd[1]: gdm.service: control process exited, code=exited status=1
Nov 02 10:46:33 niobium systemd[1]: Failed to start GNOME Display Manager.
Nov 02 10:46:33 niobium systemd[1]: Dependency failed for X11 Display Manager.
Nov 02 10:46:33 niobium systemd[1]: Unit gdm.service entered failed state.
Nov 02 10:46:35 niobium systemd[1]: gdm.service start request repeated too quickly, refusing to start.
Nov 02 10:46:35 niobium systemd[1]: Failed to start GNOME Display Manager.
Nov 02 10:46:35 niobium systemd[1]: Dependency failed for X11 Display Manager.
Nov 02 10:46:35 niobium systemd[1]: Unit gdm.service entered failed sta

GDM shouldn't be involved at all as I was using kdm.

systemctl -l status gdm.service

● gdm.service - GNOME Display Manager
   Loaded: loaded (/lib/systemd/system/gdm.service; enabled)
  Drop-In: /run/systemd/generator/gdm3.service.d
           └─50-gdm3-$x-display-manager.conf
   Active: failed (Result: start-limit) since Sun 2014-11-02 10:46:35 AEDT; 14min ago
  Process: 1405 ExecStartPre=/bin/sh -c [ "$(cat /etc/X11/default-display-manager 2>/dev/null)" = "/usr/sbin/gdm3" ] (code=exited, status=1/FAILURE)

Nov 02 10:46:35 niobium systemd[1]: gdm.service start request repeated too quickly, refusing to start.
Nov 02 10:46:35 niobium systemd[1]: Failed to start GNOME Display Manager.
Nov 02 10:46:35 niobium systemd[1]: Unit gdm.service entered failed state.

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The solution:
Either way, I now had enough keywords to start googling, and came across http://forums.debian.net/viewtopic.php?f=10&t=118305

Basically,

sudo apt-get purge nvidia-*
sudo apt-get purge libnvidia-ml1:i386 nvidia-smi:i386
sudo apt-get install glx-alternative-mesa
sudo apt-get --reinstall install gdm3

Launching gnome (standard) from kdm gave me prompts to set up the configuration for an installed extension (I miss the frippery extensions...). Not sure whether the breakage was related to the extension.

 Either way, everything is working properly now.