Joe, my interest has waned also.  Not just in the challenges but in Bryce also.  I still consider myself just a hobbiest that only learned how to use half of the program's power.  The unresolved bugs and quick releases of new features that don't work for a lot of users is discouraging. I can recite "save often because you know it is going to crash" over and over but still I will get on a roll working on scenes and mats only to encounter a fatal error at the height of my craziness.  Bummer.  Often I turn the computer off never to return to the project; the spark of the moment is gone.

Rosie and I discussed the downward slide in forum and Bryce interest a year ago.  One of the things we thought missing were the mini-tuts and mini challenges within the forum; not just the formal bi-monthly challenge.

I best liked the challenge themes that focused on specific goals in Bryce --challenges that forced participants to explore more focused possibilities. 

The abundance of content from Daz and other sources has destroyed the need and desire for users to experiment.  When is the last time someone in the forum even mentioned that arcane realm of DTE? 

So in conclusion, I only half agree with your assertion that the Bryce challenges are a way that forces you out of your comfort zone.  Too many challenges don't require one to move beyond deciding what item to select from a long list content then apply it to a scene. The most difficult part of the challenge is simply thinking up a scene rather than its execution. (my opinion)