Thanks. I appreciate the points.
I think for me it's that, just about every requirement will relate in some way to function.
Security, for example, can't just be a non-functional requirement because many security requirements actually change how the system works. At its simplest you have to build a functioning login page, rather than not have any login page. Likewise, with PCI DSS, you have to change how an operator speaks on a phone line, the equipment they use, what functions that equipment is permitted to have (can't be a wireless headset, and there can be no function to write the number down on an electronic notepad).
In one of the other replies, someone corrected me because I was using too narrow a definition of "non"". Well, that reminds me, it's the same with the word "function"". Function has very broad meanings. It's not just what something does, it's also it's purposes, and so on.
I think it would be clearer to simply name things as aesthetic requirements, security requirements, market requirements, cost requirements, and so on.
I don't see what the term "NFR" adds that's useful...?
There's far more overlap between aesthetics and function than there are differences. The example of a house, if you choose steel frame or wooden construction or something else, that dictates the aesthetics. And if you choose a different aesthetics, then that's going to dictate the construction. Even the colour of paint can perform a function, climatically, are you reflecting light to keep cool? are you causing glare? Etc.
I think there are huge overlaps across all "functions", but NFR implies they can be thought of in isolation, and I think that is misleading.