SDDM doesn't have feature parity to GDM, KDM, or LightDM, but is just focusing upon being a lightweight display manager that gets the job done for most Linux desktop users while being very slim in terms of code size and dependencies. SDDM just depends upon PAM, Xlib, and Qt but none of the KDE libraries or anything else. This new display manager forgoes any support for remote connections, XDMCP, and other advanced functionality supported by many of the alternatives albeit seldom used by a majority of Linux desktop users.
This Turkish developer took to writing his own Linux display manager after facing the complexities of KDE's Display Manager. "I know for a long there was a high demand in creating a QML interface for KDM. Since this never materialized and KDM codebase is huge and somewhat complicated for me, I took the alternate route and wrote my own display manager from [scratch]."
Right now SDDM is using QML1/Qt4, but as QML2/Qt5 deployment is more widespread, the display manager will likely be transitioned. "There are no restrictions imposed on theme, you can do whatever you want. But it has to be in Qt4 aka QML1 for now. Qt5 porting will done after qt5 gets a bigger install base and easier availability. After that you will be able to use gpu shader/effects in the display manager. Yaaay!"
Find out more about SDDM from this forum thread, including the link to fetch the sources.