From Fedora Project Wiki
Switching from KDM to LightDM login manager
Synopsis
Pro:
- Smaller code base
- Better performance (according to their presentation, my experince so far is not that great - mbriza)
- Simpler API for creating greeters
- Powermanagement is present
- Multi-head support is better - Prompt is displayed on the screen where the mouse cursor is
Con:
- Switching users in KDE is not possible. [1]