I've left Ubuntu quite a while ago so here's my biased version: I'd expect my usb wifi key not to work, to suffer to get my 9 series geforce card to run full speed, to have to fight each program independently to set up my preferred i18n configuration (English language but Italian keyboard with Italian number format and iso date format), to have to mess with dependencies to get a recent official java version running.

plus weird specific things, like at some point I had some issue with 15.04 having different security defaults requiring docker to run with some additional weird flag to let 32bit app run in the 64bit host, something which worked on 14.04 out of the box.