Have you ever read a story where the concept was amazing, but the actual book was only somewhere between 'decent' and 'fairly cool'

All the time. I have a friend who love DragonballZ. His descriptions are amazing. When I finally watched it, I found it couldn't live up to what I'd heard, so I went back to hearing about it from my friend. It's a much better show that way.

consistency of artwork is important in a visual medium

Stay the hell away from Marvel, then. Marvel are about the money. They hire amazing writers and Do. Not. Let. Them. Write. They will chop and change artists and writers mid story arc and it's best we not mention that they recycle the same tired old handful of stories every few years. Idiots!