Just to add my two cents, technically in most cases we're not talking about donations. Donations are given to charities, and most Minecraft servers are not charities. Legally, in most places, they should be called either gifts or purchases. It doesn't matter much for smaller servers. But, if you notice, bigger servers never call these donations. Donations are tax exempt, and thus could be seen as an attempt at financial fraud. If you call your ranks donations and were the unlikely soul that received he wrath of an angry "donator", they could sue and you would lose, as well as possibly be investigated for all of your server's funds.
*Disclaimer: I am absolutely not a lawyer*
