I think the biggest point is that silver age comics were written more specifically for children, modern comics are written more specifically for adults!
Silver age and Golden age comics are great on a pure nostalgia basis, but certainly were not better written or drawn than modern comics.
How many Superman stories in the silver age revolved around some stupid plot where Supes or one of his cast, were mutated into some ridiculous thing or another, and the same story would essentially be repeated issue after issue, with slight variations!
The Dark Knight and Watchmen really kickstarted an era of storytelling that actually appealed to adults rather than appealing to the child inside of them. (although I would say that stories like Crisis on infinite earths, had a lot to do with it as well)
