Wireless and graphics still cause major heartburn on a lot of devices. For example, laptops with hybrid graphics (dual, switchable cards) still don't work properly after multiple versions of bumblebee/ironhide. Just last week, I spent all night trying various ways to install a wireless adapter. The adapter came with drivers for OS versions as recent as Windows8 and as old as Windows Me, but the linux version only had kernel 2.4. I got it working finally using ndiswrapper. After that I performed an upgrade to Ubuntu 12.04 and that killed it.

Ofcourse, all of the above was really just manufacturers not open sourcing or updating their drivers. But I think the linux ecosystem is direly in need of more device manufacturers taking it seriously for it to gain wider adoption. Canonical is doing a great job but we are not there yet.

Ubuntu: 95% no problem with wifi, a couple of local hotspots have strange redirection pages that won't work properly with Ubuntu 12.04/Debian/Centos (Firefox/chrome) but which will work with Mac OS, Kindle and Win. I just avoid those.

Displays: no problem with projectors or 1080p monitor.

12.04 fixed my multiple monitors issue, but for a while it took around 2-3 hours of setup to get dual monitors working with my AMD/ATI card.