short answer - probably xorg 1.18 http://sparkylinux.org/forum/index.php/topic,3417.0.html
"My guess is that non-free drivers will take longer to transition. "
In these situations, your first step is to get into tty1 via "Ctl-Alt-F1" log in as root and gather data from there. It is possible that you might have to escalate via going to rescue via your grub options or even chroot in via a "live-cd" but your first step is to try tty1.
From some people who dealt with xorg 1.18 in unstablehttp://forum.siduction.org/index.php?topic=6032.msg49791#msg49791
Please post output of "inxi" and inxi -G"
You probably want to examine "journalctl -b -0 and for less output journalctl -b -0 | grep <<the name of your video driver as found via inxi -G>>
Any talk about this on #debian-next on OFTC or on debian forum?
My guarded opinion is that lightdm is not at all part of the problem. lxdm is a huge regression. The most probable solution is to remove the non-free nvidia binary and go with the open source debian version. It sounds like you have done that. That can be confirmed via the inxi -G
If you are doing sparky lxqt it is wise to follow the #lxde irc on freenode The #lxde irc onOFTC is geared towards LXDE