This is a great thread. People seem to still be arguing about what are riffs, licks, etc. Maybe some of you play rock guitar, I don't know. But, speaking as a rock guitarist, generally we consider a riff to be a repeating phrase that is played as single notes. Most rock riffs are built on the blue's scale, but they don't have to be. A "lick" is a non-repeated phrase, usually part of a guitar solo. Example, "Man, that was a mean lick you played in the middle of
that solo!"
Song's like "Wild Thing" are just chord progressions, in this case I-IV-V-IV repeated over and over. Rock guitarists don't really consider those riffs, although, if you look it up in a dictionary, a "riff" can be a repeating set of chords too, so, technically, about everybody is right here.
In summary, rock guitarists think of riffs in terms of the following examples;

Led Zeppelin --(definitely one of the lords of riff), "Heartbreaker, Black Dog, The Ocean song, Misty Mountain Hop, Dazed and Confused, etc. etc.
Hendrix-- Manic Depression, The opening of Purple Haze, etc. etc.
Cream-- Sunshine of your love, Crossroads, etc.

Keep 'em coming, it's fun to hear what people like in riffs.
What makes a great riff? It's your interpretation. Either it grabs you, or it doesn't. What amazes me is how many different ways a few notes can be arranged. The more unique, the better as far as I'm concerned.