For any other creator, Thor would've been enough. It's a four-year masterpiece, one that I've frequently called the single best run of superhero comics of all time thanks to its perfect blending of comic book action and the high fantasy of Norse mythology, and that's not a really difficult argument to defend. There are issues with bone-shattering larger-than-life battles, and there are issues that hit so hard emotionally that I still get a little choked up thinking about them, and there are issues that do both at the same time. Thirty years later, it still holds up as an unparalleled high point of the genre, and for any other creator, that would be enough.
For Walt Simonson, born this day in 1946, it was just the tip of the iceberg.