Personally I find making Bios/History for a CS/NS to be it and also finding the time is especially annoying. Even more so when there's a bunch of RPs or Int checks that are going and you want to join ;-;
finding the time is especially annoying.
GMing. Without a doubt.
In particular, dealing with fools who think their character concept is nice and balanced and should be approved, when it is blatantly obvious it is overpowered and unbalanced and violates half the lore.
Keeping the plot moving along is also difficult, especially when you've got many players with disparate schedules and the IC plot is chaotic in layout.
What Ellri said, while anyone could technically be a GM. There's not all that many good GMs. Personally, the hardest part with RPing for me has almost always been dealing with people. Those that aren't interested in RPing with me, but claim to be. Those that seem a lot more interested in arguing and starting drama than doing anything else. Lately it's been particularly hard to remain motivated to RP at all, and I've asked myself more than once why I still do it. All thanks to having to deal too much with certain kinds of people or behaviour. I still got that one good fun RP I've been doing for almost a year now, and that seems to be the only reason I didn't quit at some point.