When developing an RFP on behalf of my clients, one of the items we'll discuss is "nice-to-haves" vs. "must-haves" on the list. That is, identifying which functionality the system must absolutely provide, or otherwise it cannot be considered.
This activity can actually be more difficult than it sounds. For example, working with a client recently, one of the functional requirements we identified was the ability for companies to be able to do group registrations. That is, one person from a company could go online and register multiple individuals from that company for an event. As we discussed whether or not this was a "must-have" functionality, we determined that, in reality, group registrations account for only about 10% of total registrations. That is, they're the exception, and not the rule. So while this functionality would be nice-to-have, it wasn't must-have.
On the other hand, allowing individuals to come online and register themselves for a meeting (functionality the client did not currently have) was a "must-have." We wouldn't consider any system that couldn't do this out-of-the-box.
As noted above, a simple guide to keep in mind is whether or not the functionality represents the rule, or the exception to the rule. Sure it would be nice to have "invitation only" event registration so that our board members can register for board meetings online wihtout staff intervention. But since these board meetings only happen once per year, and there are only 12 board members, is this a "must-have" or a "nice-to-have"?