One thing that is for certain is that quite a lot of games have a larger margin for error than older games. Not just taking more hits, but also saving and unlimited continues/lives. If you got game over on super mario bros. you started from the beginning, but if you got game over on super mario galaxy, you only move back to the beginning of the star you were getting.
Also, over time you are going to get better at games. However I disagree that it would be the reason that games seem easier, they are getting easier, but people getting better at games would be part of it. I think games are getting easier, and I still can't beat rayman 1.
Also, as many people said, the 'difficult games' were gone since the arcades (and arcade ports) died, with few exceptions. When people look at games nowdays, it's not about the challenge, it's about graphics, story and good controls.
Also, to do with controls, controls are more consistently getting good. Let's face it, sometimes bad controls is the reason why some games were difficult back then. However, there were plenty of good controlling games that were difficult.
One more thing that I want to add. Many 'nintendo hard' games weren't really that hard after you spent the time to remember all the levels. Honestly, you can only equate difficulty to how much time spent to beat something. Once you can do it without the stroke of luck you had to beat it the first time, you will be able to consistently do it again and again with no problems. It's only really difficult the first few times you do it. Muscle memory is pretty cool I guess.