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.