I believe that while the games are becoming more and more visually appealing, it is also growing more family-oriented. Problem is that many video gamers are more solo players, and so Mario games have been rather boring for hardcore players. Even on the solo games it has been easier to play through so that gamers that probably just picked up a video game could beat it.
Now, Mario has always been the more family-friendly series of Nintendo, but I believe that older games are a lot more interesting for more advanced gamers since they find a challenge to try to best instead of simple and 5-seconds to beat.
Stories and plot have been rather symmetrical, almost like a bell-curve, but it has been denying games a respectful plot of recently like they did early on in gamer, but again, it depends on the player's interest of storyline and gameplay.
Basically, older games are more difficult via how complex some stuff are (good and bad) and have a lite more pixilated graphics. On the other hand, newer games usually have a more simple concept but really good graphics.
Note I am talking about Mario games. When it comes to nostalgia, I go for more of the middle ground because I believe that a plot runs the game. I'm not always interested in just playing a game straight through unless there is a hook I can attach to so that I can have a background story to further pull me into the game.
I think Nintendo has done a great job at trying to expand the age range of players, but I think they also have lost some fans' attentions due to a lack of games to those players to dive into.
My personal question is why not moderate it? Based on DKCR, they have the ability to make games more difficult while still being reasonable. Zelda still shows it can have a very dramatic plot as seen in Skyward Sword. They have the potential, but they continue to spread the "family-orientation" style to all areas of Mario - and some really should have quite the opposite.