Plasma

Application version: to know if our users are up to date
Compiler information: to know if issues in non-mainstream compilers are relevant
Platform information: wayland or X11?
Qt version: how different versions of Qt and Plasma interrelate
Usage time: are users passing by or they use it as their main system?
OpenGL version and vendor information: learn about our users' hardware, can we trust users to have proper 3D acceleration? Which drivers should be testing? Do people prefer open drivers?
Locale information: Are people from different locales struggling at using plasma? Are we not getting bug reports because they just don't consider adopting it?
Screen parameters: how important is multiscreen for plasma?
Panel Count (Plasma Shell): are people playing with their panels adding new ones?
Application Source Name (Plasma Discover): where do users prefer to get their software from?