There are a few annoyances, all interrelated, and I'd love to talk about all of them, but you said the most annoying thing, so...it's that things frequently don't work, or don't work totally correctly, and the only way to fix them is to be intimately familiar with the innards of the OS (and I don't care about some arbitrary OS to spend that much time studying it) or to do research, which can take any amount of time, and sometimes you just gotta resort to asking Linux experts for help and hoping they're nice.