This question on Marvel Editor Tom Brevoort's Formspring got me thinking.
Q: "It's been a number of years since it happened but I'm curious what your thoughts are on Norman Osborn. Did it turn out to be a good idea to bring him back or do you think it was a mistake?"
A: "Lemonade. I think bringing him back (and a number of other characters who had been dead more-or-less permanently) was one of the things that really damaged the credibility of death in our stories. But that said, once he was back, we told a bunch of really good stories with him, so there's that."
We all know Norman was brought back to get Marvel out of The Clone Saga. They needed somebody big enough, bad enough, and around long enough to manipulate everyone. And they needed somebody who knew Peter was Spider-Man and wanted to do such a thing (so saying "Dr. Doom did it" wouldn't have worked). Since Norman's return, he has left a major impact not just on Spidey but on the entire Marvel Universe. Some highlights include "Death In The Family" (marked in the trades as "Return Of The Goblin"), Mark Millar's Marvel Knights Spider-Man, the Warren Ellis Thunderbolts, "New Ways To Die," the entire Dark Reign era, Dark Avengers, Siege, the Osborn mini, and the Bendis "Return Of HAMMER" story.
Was the trade-off worth it? I agree with Brevoort that bringing Norman back did damage the credibility of death in the Marvel Universe but that they did get some good stories since they brought him back.