"...How do you make someone stronger than the strongest person? It finally came to me: Don't make him human make him a god..."
This has been commonly interpreted as Stan deciding that he found a way to come up with a guy stronger than the strongest "human" character. However consider this expanded discussion from "Origins of Marvel Comics", by Stan Lee:
On the origins of the Hulk:
"I had pretty much decided to let our second Marvel-style magazine feature someone with superhuman strength. But there had been, and still were, many such characters at that time, with National Comics' Superman as the first that comes to mind. Certainly, there would be nothing terribly original about someone who had the strength of Superman. But that's where the fun came in. It would be my job to take a cliche' concept and make it seem new and fresh, exciting and relevant."
On the creation of Thor:
"Look at it this way: Suppose you had a newly created stable of superstars which consisted of a teenager who could burst into flame and fly through the air, a stretchable scientist with skin like Silly Putty, his ofttimes invisible lady love, and a multimuscled misfit with lumpy orange skin-to say nothing of a wall -crawling Wunderkind and a jolly green giant-what in the name of comicdom assembled would you do for an encore? Sure, we were always striving for variety, but now it was getting ridiculous."
He continues two paragraphs down:
"But what was left to invent? Who could be stronger than The Hulk? Who could be smarter than Mr Fantastic? We already had a kid who could fly, one who could walk on walls and ceilings, and a female who could fade away whenever danger threatened-or whenever the artist ran out of ink. As you can see, we were hooked on superlatives at that time, always trying to come up with characters who were bigger, better, stronger. However, we had painted ourselves into a corner. The only one who could top the heroes we already had would Super-God, but I didn't think the world was quite ready for that concept just yet. So, it was back to the ol' drawing board."
I think these comments give a different interpretation of the oft-quoted "how do you make someone stronger than the Hulk" dilemma. Does anyone really believe that Thor is or was intended to be smarter than Reed Richards? Does it seem like thats what Stan is saying that was his intention when he created Thor? Or is he saying that the driving idea behind making a superhero who was a God was about making him stand out as someone unique in a world populated by characters who had already claimed the title of the strongest, the smartest etc. This makes Thor unique and doesnt impinge on what made these other characters special. It also seems to match up pretty well with what he actually wrote as Jonothanos points out.
As you can see from his discussion about the Hulk his primary concern when introducing new characters to the Marvel Universe seemed to be focussed on making them "seem new and fresh". Thor being an actual God accomplished this whereas making him the strongest or smartest guy at the expense of an already established character would not.
Quotes are from
http://forums.comicbookresources.com/showthread.php?p=12084758
