A lot of community or public servers will, in some way or another, alert you that you have the opportunity to donate and support the server financially. Some servers even have incentives, or extra commands and privileges for those willing to pitch in their extra buck.

But... I don't think this should be the case. I do think that people should be given the option to donate to the server, but this option shouldn't be screamed at in their face, or run on the announcements repeatedly. "DONATE TO THE SERVER FOR VIP RANK", for example, is sometimes the sort of thing you'll see the console announce on a certain timed basis. And some servers even have whole rooms or sign boards near or at the spawn, dedicated to informing you about the many ranks or various commands you can earn if you donate a certain amount of money.

To me, pestering people for donations can annoy them, scare them off, or make them feel bad about playing. And they shouldn't use extra commands as an incentive, and they especially shouldn't give $20 donators admin-like powers, or operator status. This can also-depending on the way the nag is presented, give the impression that the server owners embezzle the money donated to them, and the donators embezzle the powers they're given in return, telling players the server is run out of greed, not out of just having fun with Minecraft.

Not that it would ever go that far, because I encourage people to donate to a server, but ONLY if they truly like the server. A server should never announce to a guest or nag them to donate, a guest or regular on the server should speak up and ask, "hey, can I donate to this server?" Or a mod can suggest with subtlety if they can donate to the server. And extra privileges should come with TRUST, not with cash.