Thor's depowering goes back way before Quesada. I personally attribute it to two things: 1) his being an Avenger for so long and 2) the rise in the Hulk's popularity. It's easy to maintain a high power level when everything is tailored to the hero in his solo series. But uncreative writers on Avengers were saddled with Thor as a member of the team and couldn't come up with decent threats, so Thor came off looking like a wuss so he wouldn't (rightfully) beat many of the Avengers' enemies all by himself. Meanwhile, the Hulk received a TV series in the '70s and his popularity shot up. Thor never got anything similar because his backstory isn't as easily digestible to the masses. Then a really cheap, pathetic version of Thor appeared in one of the Hulk's TV-movies and was totally worthless, tainting a whole generation's perception of the character. So Thor's popularity fell while the Hulk's rose and, unsurprisingly, the Hulk started getting better showings in his fights with Thor and then ultimately started winning all of them outright. Only recently has Thor finally been slowly climbing back up to his level of respect as one of the top characters within the Marvel universe.