I remember the iRiff contest getting really heated. Overall, while the thought of consulting with "the guys" was hugely tempting, I think it was a negative. Certainly the response to it seemed like a headache to deal with, so I don't blame the site for never mentioning it again.
Riffing is hard to do. Our big obstacle was (and continues to be) all the tech aspects of it. Keith and I had never done any recording or sound editing before this, and it shows. Our early rifts sound pretty bad, with inconsistent volume, clipped lines, this weird buzzing noise that took us forever to figure out how to fix, sync lines that would drift so that THEY were off even if the track itself was synced, etc etc.
Point is, riffing is a lot more work than it seems at first, and most of it isn't the sexy, joke-writing kind of work. I think a lot of people just never got over that first hurdle and quit. So there are a lot of early rifts or one-off rifts that are just hard to listen to, and I include our own early efforts on that list. (I hope we have improved since then.)
But tech issues, while they can make a riff very difficult to sit through, are still forgivable. Provided the issues improve, I'm willing to give groups another chance, and sometimes I'll just deal with the imperfect sound quality if the riff is funny enough.
The jokes are the important part. If they're not funny, that's it. I think you can definitely improve with practice, but up to a point; either you have a style that I'll laugh at or you don't. Humor is subjective, and I think you can be proud if you make even one person laugh at your riff, but no amount of grinding is going to make me like your jokes if I didn't like them already.
One thing that does bother me, and I don't mean to offend if this is anyone's style (I don't think it is), its the people who do a riff by just hitting record and going off the top of their head. The jokes are rarely great because they were just immediate, not much thought was put into them, which I think leads to jokes that are funny to YOU but not to total strangers. Plus, doing it that way doesn't account for any tech issues that might pop up, so usually those riffs also suffer from inconsistent audio, drift, and a whole mess of other problems.