I'm with Stewart: lethargy, mostly. Also, I need a purpose for my pictures. That doesn't mean that every single picture is shot for a reason, but I have to have some sort of`publication in mind.
After that, it's just practice, practice and more practice. I don't necessarily agree with the OP that " One must have the ability to visualize the image mentally, then frame it and shoot it," though. That's what the viewfinder is for. Sometimes I visualize a picture. More often, I see it and shoot it. My most successful pictures normally come from the latter procedure.
It doesn't matter much what equipment I use, so I like my gear to be as 'transparent' as possible, i.e. I want to spend more time on framing the picture right than on working the camera, and I don't want to spend much time on either. Most of my best pictures have been "see it; shoot it."
EDIT: Oh, and one more thing: impatience. The longer I have to wait before I think it's the right time to press the shutter release, the less happy I am.
Cheers,
R.