There's no question that the games of the past were great. Amazing. Stuff of the gods. But the main reason why some think that current games aren't nearly as good as their 2D predecessors might not be that they're just not as fun.
Well, technically fun IS the key factor.
Let's face it, when the original games came out, they blew everybody away. No one could comprehend the shear awesomeness of a video game. People were ecstatic. People waited in lines for hours on end for the new Super Mario to be released, with it's tiny cubes of color, lasting even more hours than those spent waiting. The hype was incredible.
The generation who loved those games might not like todays games because they don't bring the same excitement. Not to say that these new games aren't exciting. I personally waited in line for a good few hours to get my copy of Halo 3: ODST and loved every minute of it. The somewhat older group of gamers have been around since the first "Glory Days" of gaming, so now it may seem like they've seen it all. Newer games just don't bring them the fun that Zelda, Super Mario, or, hell, even Pacman, brought.
I myself think games can only get better. I grew up with the Sega GENESIS and N64 and they were the coolest things I ever say at the time. Since then, my experience with games and gaming as a whole has only improved.
The fun and excitement is still there. I for one, haven't lost it. Gaming will keep getting better and better from here. Hopefully everyone who thinks that the quality of games has been lost will be able to pick up a controller and find that quality once again.