'NES hard' refers to a time when most games had around an hour of gameplay at best and they needed to stop you from experiencing it with tons of intense difficulty spikes, limited lives, no continues, infinitely respawning enemies, death pits, infinitely respawning enemies with death pits (Ninja Gaiden), etc. They were designed this way so that you couldn't just beat a game in a weekend and return it to Hollywood Video, the arcade experience at home.
The reason why games like Mega Man, Mario, Zelda, and many of Nintendo's other offerings are considered classics is that it was actually possible to fucking beat as a kid without banging your head against the wall at one specific point for hours at a time. It's why shit like Battletoads, Ghosts and Goblins and Double Dragon are relegated to XBLA games instead of major franchises and other series like Final Fantasy continued through to future generations