I'm still interested in Barty's point, though - there does seems to be a step-change in what you expect rappers to sound like over about the first half of the nineties, even if that doesn't stop me getting into Rakim. If I was going to try to pin it down I'd say it's that the flows get looser and less locked on the rhythm and rappers make more of a feature of imprecise diction, accents, nonverbal noises.

It's at the same time that slow beats get a lot more prevalent and explicit gangsterism gets more mainstream, both of which are presumably related on some level.