Personally, I want to see economy. I've been toying with the idea of a server running Gregtech, no item or fluid teleport, and capped world size. I imagine that it would force players to trade, and cooperate, especially for high tier builds (like a server shared fusion plant), and would force players to live closer together, more in cities / towns where they can take advantage of shared builds and resources. I was also thinking about dumping in all of the "hard mode" food and mob mods as well, to give players more of a reason to build defended and walled cities.
As for servers, I think it really depends on the community that you have. From what I've seen, servers work best if they're like Forgecraft, where everyone knows each other fairly well, we access is controlled, and everyone is fairly mature. There are simply too many ways to grief to protect against, other than walling yourself off in a mystcraft age and calling it good.
As to servers selling items, I agree, although I do think that some sort of donation reward should be allowed. On one server I was on, one of the benefits to donating was that the admins would make a small (three Chunks square to start) island in its own dimension for you, with the size of the island determined by how much you donated. It was a public (with application process) server, so a lot of people donated to ensure that they wouldn't get griefed.