One of the reasons he's disliked is because people have still in mind the Silver Age version of him. Even though for every other character people think in their current form (for instance, when discussing Batman people will talk about the dark-brooding version and few will say they like him for how he dances the Batusi and onomato-punches people). Case in point: people in this thread keep saying how he's invincible and only has one weakness, which couldn't be further from the truth. Heck, he's probably easier to kill than Wolverine.
Every character once in a while gets updated to keep up with modern times. If people can accept that for everyone else, why do they keep Superman locked in the 1970's pen?
Another reason is that people are not objective when comparing the stories. When every superhero succeeds, either Superman or any other, it's not because they're all-powerful, but because the writers are. How many times have heroes survived out of convenience rather than skill? How many times Batman or Spider-Man have survived because their foes decide to capture them and distractedly give them a speech instead of just kill them? Why no one uses a sniper rifle to blow the Punisher's head off? In those cases, the heroes' skills mean nothing: they survive because the story demands it. Superhero stories are supposed to keep them alive, Superman stories are the only ones that don't try to pretend otherwise.
Furthermore, I don't think Superman is anywhere less realistic than, say, The Hulk or The Flash. Even though their stories involve a human gaining abilities through an accident rather than being an extraterrestrial with a convenient human shape, you have to admit those are pretty outrageous accidents. Even Stan Lee himself, while discussing The Hulk's origins, admitted he couldn't "tell a gamma ray from an eggplant", but he wrote the story that way because "it sounds good".
Thing is, pseudo-science might sound more realistic than magic or alien powers, but it isn't. You can't expect us to honestly believe Spider-Man is more realistic than Thor, because while Spidey's might sound like a legit origin for people who know absolutely nothing about science, it definitely isn't. Heck, even outside the origin things don't ring true, as someone with the proportionate agility and strenght of a spider would be weaker than a normal person.
It all boils down to the stories, and if you're going to only consider those stories in which Superman stops a couple of thugs from robbing a jewelry store and those in which Spider-Man has to come to grips with his best friend being into drugs and turning into a supervillain, you're being absolutely unfair. Either you pick good stories for both or you pick bad stories for both.
Honestly, people keep saying how being Superman is too easy, while I think it's really hard. Not only he has to deal with overcoming the temptation of having so much power and not using it for evil, but also with keeping himself at bay in order not to accidentally hurt someone. Try to kill a fly on a glass table with a sledgehammer without breaking the table and see how easy it goes. Now imagine your entire life being like that.
Also, I happen to like heroes who help people because they honestly want to and not because they feel guilty or because they're forced by the circumstances. Sure, the latter might be more relatable, but who's better to look up to?
