I think the people saying that it doesn't matter since the mainstream Spider-Man is still Peter are all missing the point. I can't speak for everyone who hates this change, but personally I just don't like that this is a character we have never seen or heard of, and is only being made black/hispanic in order to be ethnically diverse- something that even black and hispanic people are groaning over in these comments. While I don't personally like having characters 'taking up the mantle' in comic (With some exceptions), I technically have nothing against a Spider-Man who isn't Peter Parker, or a Spider-Man who isn't white. If this guy had been introduced to us before now, and had been established as one of Peter's friends that could conceivably take up the mantle, then fine. Or even if they had decided to make this into an Ultimate version of Miguel O' Hara and his villains/supporting cast, that too could have been cool.
But nope, brand new character, brand new supporting cast, most likely brand new villains...a superhero is so much more then a costume or a mantle. The person underneath the mask is what makes a superhero who they are. Peter Parker IS Spider-Man, Bruce Wayne IS Batman, Clark Kent IS Superman. It's not just powers and tights. It's why I never actually felt like I was reading Batman when he was replaced with an Iron Man/Punisher wannabe, it's why I never felt like I was reading Spider-Man when Ben Reiley took over, it's why why people hated Frank Castle as Captain America. It just isn't the same without the people who defined what it is to be the specific hero. Maybe Miles will end up being a really great, fun character. Maybe his supporting cast will be the best thing to ever happen to comics. But without Peter Parker, without Mary Jane or Aunt May or Gwen Stacey, without Jameson or the Osbornes to turn Spidey's life into a living hell, it just isn't Spider-Man. It's somebody pretending to be someone else, especially when they have no ties to the supporting cast whatsoever.
Further, if they wanted to make a superhero with a more ethnically-diverse cast, then why not just make a new super hero? Answer: because only hardcore comic fans will hear about it. Marvel wants new readers, and they want to appeal to a larger audience. Since no one will hear about a new super hero, they have to do it with a popular mainstream hero that everyone knows...like Spider-Man. A move like that is more likely to gain attention in the media then a new hero ever would.