For me its gotta be photographing my models. They just never look as good in pics as they do to my eyes on my bench. Part of that I believe is the kind of love you have for something you've built and poured hour upon hour into. You become enamored with your creations, and focus on the best parts and not the flaws.

The camera can strip all that in away, and flaws you missed become completely obvious under the glare of the lights. I've been using a 6mp point and shoot, does a DSLR really make the difference? I know my lighting isn't the best either. Setting up a backdrop, getting the model framed and in-focus correctly, with the lighting accentuating the best features of your model, that's hard.

These are the things I find most challenging and I would like to learn how to do it better (without ponying up ~$500 for a DSLR and lights, backdrops etc).