After I've been meddling with xrdp for quite a while, trying mint, xfce4 and xubuntu as xrdp desktop environment, I have my login dialog changed to this. I can neither tell which desktop environment part it is nor what package should I remove or reinstall to fix it.
This article helped: Restore Unity Greeter
Now on a new install of Ubuntu 14.10 + xubuntu-desktop on top of that, there is no directory lightdm.conf.d under /etc/lightdm. But following advice from here I created the directory /etc/lightdm/lightdm.conf.d and a file /etc/lightdm/lightdm.conf.d/50-myconfig.conf with contents
and I get the unity greeter back.