What is it about him that makes him so special? For me, it's that for his first 30 years of existence he always had at least one creative team delivering the goods. No other character(s) have been fortunate enough to have such a solid base be given to them.
He is special 'cuz he was bitten by a radioactive spider and now has the equivalent strength of a human-sized spider AND nifty-tingly sensations as well.
Yeah, he is a well-written character for so long is due to the fact that he has been a well-written character for so long. It's probably something to do with his tightological(?) web-spinning.