I don't find him overpowered. And there appear to be enough comments on this thread that indicate to me that people with keener eyes see that as strong as the Hulk is, he is really outmatched by a few others at Marvel.
Having said that, a number of things contribute to the Hulk's present status as Marvel's poster boy for superhuman strength.
Editors and writer's can be fan boys too. Stan Lee certainly had in mind Superman when he created his early stable of superheroes. The Thing was the most popular character of the Fantastic Four, Marvel's first home run. So Stan figured he model his next character after the Thing.
Enter the Hulk.
Stan indicated he wanted a guy with superhuman strength, but to arbitrarily create someone with the strength of Superman (his words, not mine...) would not be terribly original. So he created someone who was a monster (like the Thing), with the strength of Superman, but modeled after Jekyl and Hyde.
Thor is as strong as an enraged Hulk.Thor was actually Stan's first attempt at a "Cape" for Marvel comics, modeled more in the fashion of Superman, without the atypical Marvel character flaws, feet of clay types that had become a hallmark of Marvel heros at that time. However, Stan felt like he had painted himself into a corner. "Who could be stronger than the Hulk? Who could be smarter than Mr Fantastic?" He felt like the only way to top the others was to create "Super-God". Then it occurred to him, while his previous creations were mortal, why not borrow from Norse mythology and make a god a superhero. And the rest is history.
Now, Stan will be the first to tell you that at first, when the Hulk first changes, he is not as strong as Thor. However, an enraged Hulk is clearly as strong as Thor. In Bring on the Bad Guys, Stan made the comment that when trying to come up with characters capable of opposing the Hulk, they were driving themselves bonkers. "He had become so incalculably powerful that after facing down The Mighty Thor, who was left?" Hence, the creation of the Abomination (who was subsequently de-powered, destroyed and replaced by A-Bomb). The evenness of the strength between these two behemoths is exemplified in Defenders #10, where the two powerhouses stand deadlocked in stance for an hour, neither one overpowering the other (and the Hulk, presumably at his his height in strength as he was already rampaging through New York by the time he and Hulk meet). It should be noted that the question, "Who is stronger?" is NOT the same as "Who would win in a fight?". Conversely, a win for one over the other of these characters should not necessarily be an indication of who is stronger. It should also be noted that Thor has stated, and others have stated, and blurbs have indicated as much, that Thor often holds back when fighting against mortals for fear of killing any of them. This would be particularly true where Banner is concerned, who is simply suffering from the curse of a gamma radiation accident. Banner is a good man. Killing the Hulk would kill Banner. So when it comes to battle, Thor usually holds back. That is why Defenders #10 is such a powerful indicator of nothing but raw, brute superhuman strength, and how evenly the two are matched. And without any concern of harming Banner (or other nearby innocents for that matter).
It wasn't until after Stan left his duties as EIC and Jim Shooter after him, that the notion that the Hulk was stronger than Thor began to blossom under DeFalco. He outrightly declared Hulk to be stronger, although, Thor was still more powerful overall, according to him.
Up to this point, the philosophy at Marvel was to create an air of suspension of disbelief. Try and make the powers and abilities of the characters somehow...believable.
Enter Joe Quesada. "It's comics! Anything goes!!" That was his motto and it appears to have been adopted by Matt Fraction (who has quoted that motto verbatim incidentally...), the current writer for Thor. With Axel Alonzo, he seems to be valiantly carrying the torch for Quesada.
A synopsis of Thor and Hulk confrontations over the years elicits a number of different interpretations from different folks. Generally, it is agreed upon by many (not by ALL), that most of their confrontations have ended in draws, although a clear victory appears for each character in Hulk Annual 2001.
So, while Marvel may currently hold the Hulk as superior in strength currently, I think they may see Thor as more powerful. Maybe.
However, a return to Stan's original vision of Thor as Marvel's strongest, most powerful character, paints Thor as superior to the Hulk in every way.
Silver Surfer is clearly more powerful than Hulk. Just about every time these two have met, the Silver Surfer has been the clear victor. The Surfer has typically been painted as more powerful than the Hulk, and the fight usually will indicate as much.