This video shows some very old concept art of Diablo and Azmodan. I think we can use the evolution of Diablo's appearance to see what Blizzard is trying to portray in his character.
Most of the pictures depict Diablo as being skinnier than we was in Diablo 2. These were drawn in 2007, which was before Diablo 3 was officially announced. At that point I doubt they would have a coherent idea of what the story would be. Although many of the pictures show him as being skinny, they all appear to be very masculine.
Because of this i think that they are trying to make Diablo's body resemble the evil he respresents. Terror is more of a mental evil than physical so i don't think they want him to be a monstrous brute. Also, if Diablo was a hulking mass of muscle, many people would assume he was stupid because that is a common stereotype, which is not the case.
Along with this, I also believe that these pictures support the theory that Diablo has merged with Mephisto, specifically the image at 2:19. He once again appears with an extra set of arms along with horns very similar to Mephistos. Furthermore, he seems to be floating, which is a trait of Mephisto, he seems to have an odd glow in him, and he is skinny just like Mephisto.
Tell me what you guys think