The line between is fine yet exceedingly important. The expression "Rock 'n' Roll" has come to mean more than just a style of music. It is used as an adjective to define that elusive quality that raises something beyond the merely great to the magnificent. Thus to say "Talking Heads are a fine band, but they're not very Rock 'n' Roll" will put the final, undisputed nail in the coffin of an argument that could rage, over several bevvies, well into the small hours. To describe a singer as having a lovely voice, but "she isn't very Rock 'n' Roll" will spell doom. A career of free lunchtime concerts and perhaps the odd minor hit single will follow, but there will never be the no-holds-barred, drag it down and rip it up adulation of an audience who are true believers and who really know what Rock 'n' Roll is

By extension you can use the expression for things that have nothing to do with Rock 'n' Roll itself. To better help illustrate the concept, here's a list of comparisons which should make things clearer. It doesn't mean that those in the right-hand column are necessarily bad, it's just that they're not... well... Rock 'n' Roll. All correspondence gleefully entered into.