Requirement: The site must be viewable from a simple web browser. No fancy clients, no plugins, not even Java. Nothing other than what you would expect to come standard given today's level of technology. The site should be preferably LynxFriendly. Failing that, it should definitely support the major browsers (see BrowserUsageStatistics).

There are of course exceptions to this requirement (NapsterDotCom comes to mind immediately), but it's a good one to follow.