That's a good question. Unfortunately I can't do System76 and Ubuntu guys justice here because I really don't know the work has been done in both distros.

From the user perspective, I would say smoother hardware integration (I download the ISO with NVIDIA proprietary drivers, haven't encountered any graphic card comparability problem), consistent theme across gtk and qt based applications , saner defaults and clearer setting structure, good keyboard shortcuts. There are merely less than a dozen shortcuts that are not found in Ubuntu, but very easy to get used to. like win + up/down to switch between workspace.

I know all these can be accomplished by extensions, add-ons and themes. But they do feel different when it is out of the box and carefully curated.