I have heard that LÖVE is a great engine for prototyping game mechanics. You know; have some colored circles and some boxes and see if your game mechanics pan out (if you're doing artwork then you're doing prototyping all wrong). Hand your prototype out to friends and family and see what they think.

Starting with the graphics, sound, etc. feels rewarding but ends up being a really slow way to fail. Your game should be fun without the polish.