"systemctl status lightdm" gave me the clue that the ExecPreStart failed, I had a different path than expected in /etc/X11/default- display-manager (usr/bin vs usr/sbin). "systemctl status" is actually really nice.
** Changed in: lightdm (Ubuntu) Status: Confirmed => Incomplete -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to lightdm in Ubuntu. https://bugs.launchpad.net/bugs/1329056 Title: lightdm does not start under systemd Status in “lightdm” package in Ubuntu: Incomplete Bug description: Lightdm does not start under ssytemd. Starting it by hand works. ProblemType: Bug DistroRelease: Ubuntu 14.10 Package: lightdm 1.11.3-0ubuntu1 ProcVersionSignature: Ubuntu 3.15.0-5.10-generic 3.15.0-rc8 Uname: Linux 3.15.0-5-generic x86_64 ApportVersion: 2.14.3-0ubuntu1 Architecture: amd64 Date: Wed Jun 11 21:08:35 2014 EcryptfsInUse: Yes InstallationDate: Installed on 2013-06-20 (356 days ago) InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Alpha amd64 (20130618) LightdmConfig: [SeatDefaults] user-session=ubuntu greeter-session=unity-greeter SourcePackage: lightdm UpgradeStatus: Upgraded to utopic on 2013-11-10 (213 days ago) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/lightdm/+bug/1329056/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp