Describe an object that would challenge the abilities of members here if they had to create it in 3D.
I don't think MODELING is the problem anymore. Which such sophisticated modeling programs using NURBs, and even current solid editing technology with improved boolean operations... I really don't think there's anything I can't model.
Photorealistic rendering is what is the big thing nowadays, I think... it used to be "who can make the best fire" and now someone made plugins for software that makes it easy... it used to be "who can get gemstones to appear realistic" and now there's plugins for that...
I'm not sure what the current hard-object-to-make is... I'm not too up on the current events of the rendering community anymore. Too busy modeling.
I don't think there's anything that's hard to model... just... time consuming. I would like to be proven wrong though, and give something that proves me wrong, a shot!