Alright so here's the thing: I love Marvel comics. Love. Love. Love. Love. Right now even, I have an unread copy of this week's Avengers Academy sitting there staring at me, begging to love it. That being said, I should make one thing very clear:
Of all the Marvel heroes, Thor is the one who should not exist. (and Valkyrie but she's basically a female Thor)
Here is my reason why.
Thor breaks the Marvel universe. Just look at the lineup that we have going into it: Fantastic Four, The Hulk, X-Men, Spider-man, Daredevil, Iron Man, Dr. Pym (Ant Man/Wasp), Hawkeye, Captain America, and many many more.
Every single hero on that list is either one of two things: A scientist who got caught up in an experiment gone wrong, or a regular (or irregular) guy who came across a strange set of circumstances and now chooses to deal with it by being a hero.
And then there is Thor. While he does technically fall into the second of the categories that I listed, the powers that he attains make him LITERALLY a god. It changes the entire mythology of the Marvel universe(s), in a way that, in my opinion, is not for the better.
Up until the point of Thor, Marvel had a really real theme going with their works (or as reals as you can in the comic book world). All of the places that are shown, with the exception of specific structures and alternate timelines are actually real world places that evolve and change around the heroes depending on the time that the comic was released.
I like Thor as a character, and love him as an Avenger. But I don't like what his presence does to the universe as a whole.