Well, modern games simply don't test your gaming skill like the "oldschool" games did. Playing RPGs like Morrowind almost exclusively, my ability to beat games like Ninja Gaiden atrophied considerably. The many gems of the NES library tested your reflexes, timing, precision, memorization, resourcefulness, experimentation, and patience. You had to develop your abilities and improve yourself as a person in order to beat these games. Modern games, in stark contrast, ask next to nothing of the player.
NES games can get pretty frustrating because they are very unforgiving. Mega Man, Ninja Gaiden, Zelda II, Castlevania, and many other games had bottomless pits as well as knockback - a killer combination, where a single mistake resulted in death. Many monsters in these games had erratic or unpredictable movement patterns, further compounding the high-stakes nature of bottomless pit scenarios.
I'm finding myself raging a lot playing these old games, and I often quit angrily within half an hour. But it's okay, because when I return to them the next day, I'm immediately much better. Modern games are kind, but present virtually no impetus to improve your skills. That's why you should play NES games for 10 minutes a day. It's better than meditation. You'll develop a razor-sharp mind and coordination that games nowadays are useless for.
This post is not to be interpreted as anti-modern gaming. Morrowind, to name just one, is an indisputable masterpiece.