I have a quite wide monitor and it's quite annoying to see the login dialog in the login screen on the far left side of my screen. I literally have to turn my head around to look at it!
Is there a way to center the login dialog?
No, the login dialog is designed to be in that location and this is not configurable.
However it would be possible for people to make their own greeter that is more configurable for things like this:
If you use lightdm-gtk-greeter instead of the default unity-greeter you can specify the login window position since version 1.7.0:
Edit your greeter conf file (/etc/lightdm/lightdm-gtk-greeter.conf) and set the position:
# 960px from the left and 25% from the top position=960 25% # or 25% from the right and 400px from the bottom position=-25% -400