Other than that, you're welcome to do almost anything you want. Your site should be easy to use and have a bunch of extra features which make it appealing. For example, if you're doing an auction site, you might want to put in a bunch of bidding history features.
You'll probably also need to put in some sort of administrative interface on your site, either over the web or with scripting languages of some sort. Which one you do may depend on the complexity of the task at hand - if, for example, you have a store, and you're just making up a new description for an item, that's easier to do with a web interface, but for emailing users about item availability, that's something better done with a script.
Here are a few ideas:
But really, you can do anything you like, as long it's substantial.
In summary: I'd like features, functionality, and a clean interface.