If you have not participated in challenges yet, what might make you want to give it a try?

I'm not going to participate in anything demanding a big investment in time. As a programmer, I really try to avoid spending all my free time doing programming when I already do that 5 days a week for a living. Plus I mostly browse SO while at work, and then I'll obviously have even less time for non-work activities.

Something that can be finished in a coffee/lunch break would be suitable. I won't touch anything that requires a lot of planning & design, then many thousands of LoC, no matter how fun it seems, because I simply don't have the time for it. What I've seen from these challenges, they have been way too complex and time-consuming. To the point where I'd normally go "ok I charge this much per hour"... I have no shortage of similar time-consuming real-world projects that I actually get paid for.

A format which encourages small solutions and a bit of quick fun would be more likely to appeal.

Other than that, something that involves actual interaction with other users would also be more interesting. If I have the urge to read tons of strange source code written by others, there's Github for that. So I'm not overly interested in other users' contributions, especially not if they are large and complex.