HDR for one thing. It took years before we got proper Optimus support, and it will probably take just as long to get HDR support. Having to use the command line is another problem. And poor support for HiDPI and multi-monitor. And the lack of a standardized stable platform for application development. LSB tried and failed, and now Flatpak and Steam looks like the best hope. Of course, that goes far beyond a DE, but I hope System76 will push for improvement.