As far as I'm concerned, good design is timeless. Mario and Mega Man games are still fun to play today because they were well-designed. Sure, they were difficult, but if you died, it was your own fault. You had all the tools needed at your disposal to finish the game.
I guess you could say they didn't contain "BS difficulty".
Compare that to Rayman on PSOne, which I recently tried to play for the first time. Beautiful, beautiful game, much better looking than either Mario or Mega Man. And yet, if you were to tell me it is one of your favorite games, I'd have to assume you're saying that because of nostalgia. The game doesn't have, and never had, good design.
It's just so difficult for all the wrong reasons. Like, they designed the levels just to frustrate you, or made sure Rayman's abilities would be nearly useless in them. He's really slow, they make the enemies fast. It's a pain to attack while jumping, they put flying enemies in there. Tiny platforms? Let's put enemies so small your attacks can't touch them on there. You can't spam your attack since you gotta wait for your fist to come back to you? Let's make sure we have enemies that take a dozen hits to kill in there.
I can go back to Mario and Mega Man, no problem. Rayman, which is more recent and more beautiful? Eff that game.
As for the topic; are retro games as fun as modern games? If they were well designed? Absolutely. Sure, today's games are better-looking and everything, but there are a tons of games out there that I find badly designed, and no matter how beautiful they are, or how vast their worlds are, I think the classics compare favorably to them and are more worth sinking time into.
And this is not me putting Mario up against shovelware, either, but against well-regarded games such as Assassin's Creed 2 and Kingdom Hearts, though I don't know if I want to get into that.
If you want to, you can probably look for my Kingdom Hearts rant here on this forum.