The man could draw absolutely anything.
His work was immediately recognizable, though he presented a wide range of moods and textures.
He had what I consider a truly civilized sense of humor: things were precise but crazy funny as well as technically impressive no matter how many times you looked at them.
He was almost magically productive. And while his style is wrapped in the ’50s and ’60s, the appeal will last forever.
Finally, he was the all-time master of drawing monsters. His EC stuff scared the bejeezus out of me as a kid, but the wondrous thing was that there was always a strong whiff of funny as well, so they were not simple terrors. Notice the Weird Mickeys below:
P.S. I would argue that while he was a fundamental E.C./MAD artist, he eventually transcended that classification more than anyone else.