Though I haven’t used (and won’t use) Ubuntu, it has a few things I appreciate (as far as I know them):

Ubuntu One: I wish we could have something similar for Debian users (ideally, it should be extended so that we could sync our calendars, contacts, liferea data etc.).

usb-creator: seems the easiest way to create a bootable usb stick containing a complete and customisable environment. I am aware of the existence Debian Live, but I must say I haven’t given it a try yet.

Desktop orientation: Ubuntu users are in a large majority desktop users. Ubuntu is imho desktop-oriented, and I particularly like the artwork coherence (from boot loader to default desktop).