The problem with longbets is that you can post a challenge without being required to accept a bet. I emailed the organization and they helpfully confirmed this.

>[Me:] "Must the makers of predictions (eventually) accept a challenger? I get the impression that several of these predictions are dubious yet have been sitting for awhile without forming a bet. It seems against the spirit of the site to allow predictors to grand stand without having to put their money where their mouth is."

>[Them:] "Predictors put their money where their mouth is by paying $50 to be published. They do not, however, have to accept challenges to their Predictions."

If it were up to me, they'd also allow bets with odds other than 50-50. Still, I think it's a great site.