Mine is without a doubt is... being too much of a perfectionist. I will not launch a product, upload a website or design unless it's 100% perfect. I tend to think I have a real eye for detail but a lot of the times it a pain in the a** when it comes to getting things done. Everything has to be perfect so it usually takes a lot longer to get things finished.
I can't even outsource a lot of my work b/c I know I won't be happy with the results; I guess on the upside it's made me have to learn a lot of elements of online business... design, programming etc on my own.
So whats your downfall?