Here's the problem with oversimplifying it to client/server mainframe terminology - people get all caught up in the fact that client/server stuff isn't "new," and thus since it went out of vogue some time back, it's disproven technology.
The fact is, whether or not the "average user" doesn't understand the difference doesn't mean there isn't one.
The average user doesn't understand the difference between a lithium ion battery and a nicad battery.. The average user doesn't understand the difference between an internal combustion engine and a hybrid electric engine. Just because different technologies appear different on the surface doesn't mean that there aren't fundamental differences under the hood.
It's important to clearly define these differences, particularly for those who are choosing to participate in cloud punditry but can't be bothered to look up the wikipedia entry on cloud computing.