Interesting side note: The dragons of "Game of Thrones" look very like what I outlined above. They have hugely muscular, deep-keeled chests tapering sharply toward the hips, and enormous, robust wings. Their tails are long and thin, and their legs are bony, more birdlike than reptilian.
I prefer my dragons with four legs, but that's a matter of taste (and it requires explaining how a vertebrate ends up with six limbs, and how the forelegs and the wings attach without crippling each other). In physique, they're exactly how a dragon built for powered flight ought to look.