A good riff should, I think, make you squint. Or wince. Either way it’s eye-narrowing. Good Pop seems to have the opposite effect, it's eyebrow-raising , eye-widening, head-lifting, literally helps you keep your chin up. A good riff is akin to a witty remark somehow, it’s partly in the anticipation of how it will resolve itself and then a kind of gratified appreciation of the rightness. I think that’s probably why lots of great riffs need to be fairly slow, timing is the essence of comedy, speed is essential to riffing, or at least the affect of the classic hard-rock/proto metal riff seems to be. Headbanging is natural response to the way it pulls you forward, demands your concentration and then rewards it. A kind of fervent nodding in approval.
This one’s even more obvious. But if it were rubbish, it wouldn’t be so famous would it?