Here is the major difference between Marvel and DC:
The Marvel universe is the world outside your window. The DC universe is a fairy tale.
My favorite character is Sentry, while my least favorite character is Superman.
The interesting thing about that though, is that I don't necessarily like Sentry for what the Sentry himself is, just like I don't dislike Superman for what Superman himself is.
I like / dislike them, because of how the universe they live in affects them.
If you had the powers of either one of these characters, in your mind (and the DC universe) you would be Superman. In actuality (the Marvel universe) you would end up as the Sentry (or Plutonian, if you're not schizophrenic and agoraphobic).
- Superman fights Brainiac to save the entire world and then makes out with Lois Lane in the end! Everyone loves him, everyone admires him!
- Sentry has to save thousands of people on day to day basis without an end to it... and he slowly starts cracking under the pressure. His wife is considering an affair, since he is never at home. Everyone fears him or wants to use him as a weapon.
My second favorite character is Spider-Man, while my second least favorite character is Batman... again for similar reasons.
- Batman was a kid, when his parents died. There is nothing he could have done against it. His sidekick died, but that didnn't matter, since he got a new one and the old one come back anyway.
- Spider-Man however lost his uncle, because Spider-Man made the mistake of being full of himself and stubborn. Oh, and his girlfriend died, because he was too bad at saving her. So she won't be cheering him up anytime soon.
Long story, short: It's so much easier to relate to the Marvel universe. The Marvel universe is far less forgiving and doesn't shy away from putting its characters into situations, where DC on the other hand would be more careful about that.
PS: However, I do think that every now and then (although very rarely) DC does it right. For example, one of my favorite stories of all time is "Superman - Peace On Earth".
I really liked Superman in that story, because in my opinion that's how Superman needs to be portrayed as a character. In that story Superman tries to do something, that actually matters - Only to ultimately fail, since saving the world is an impossible task.
PPS: Don't confuse realism with cruelty.