Well… they don’t like the design of a “system tray”. To be fair, it’s a very Windows centric idea, and the notion that they must provide one because Windows has one seems… similarly questionable to me too. Speaking personally I hate the idea, and always have. It’s a real dumpster fire because:
Lots of drivers (on Windows) assume you don’t know how to launch programs, and force a permanent launch shortcut on you.
Programs assume you don’t understand how to minimize or hide a window, and put themselves in the tray instead. (launchers, chat programs, etc)
Some programs seem to use them just to put their logo on the screen. You can’t really do anything with the tray icon.
Few icons match stylistically, and even on Windows, they don’t match the system style. (White icons on a white taskbar? FFS)
Programs often don’t provide an option to disable their tray icons, and it’s rare that I want them.
I guess I found the lack of them to be a breath of fresh air when I first tried Gnome 3 a few years ago. The current iteration doesn’t quite work though… 99% of the time I just want an option to kill the damn things, but I’ve have had some programs that only provide functions through the system tray. It’s dumb, and I hate it, but it is what it is.