I guess it varies depending on whether you're talking about language, violence or weirdness. I still remember being shocked by early episodes of Heroes in which we saw the cheerleader with her insides opened up or other characters with the tops of their heads lopped off and their brains scooped out. I was shocked because it was on NBC, not TNT or something. But the networks will be pickier about things like language. Go figure.
As for bringing back the actual old shows...that's asking a lot, considering a lot of shows are already on TV Land, American Life, etc. You'd either be depending on the audience that's too cheap (or can't afford) to pay for cable, or you'd have to come up with shows that don't appear on-air at all anymore. I admit I can think of quite a few: The Prisoner, Babylon 5, Nowhere Man, Ellery Queen, Mike Hammer, Mannix, Night Gallery....
OTOH, I have no desire to see ABC butcher Nowhere Man with animated bugs, squashed credits and material edited out to allow for extra commercials.