Here's what I like about this server's donation system: it isn't game breaking. Some servers give OP-like powers for donating gigantic amounts of money (the most I've seen being $300). What makes this server fun is that it's balanced. I don't mind people being able to buy money and experience, but anything more than that has the potential to break that balance. Things like mob disguises are the best donation rewards, because they're just for fun.
I'm somewhat new to this server, but the whole reason I've stuck with it is that the ranking and donation system isn't broken many, many other servers. I intend to donate after I know for sure that I like this server, but I won't be donating to obtain anything specific. I'll be doing it because I like the server.