I've always thought the original series was the best, even better than the Mega Man X series. The NES games were challenging, despite their relative simplicity. Yeah, some enemies and bosses were ridiculously hard, and jumps appeared downright impossible—the disappearing “vacuum” blocks come to mind. But the beauty of the games was that you could do a lot with just shooting, jumping, and sliding. Capcom made the most out of the limited core skills Mega Man had available—not to mention the audio hardware of the NES—and as a result made one of the best action-platformer series in the history of video games, which will soon be nine games strong.
Here's some good news you might not have heard yet: Mega Man 9 is being made by the same core team that created the first batch of Mega Man X games, so you know it's going to be in the right hands. Though I have a slight worry that the difficulty will be toned down somewhat to cater to a more casual audience, my gut tells me that it's going to be just as challenging as the NES games. Konami didn't pull any punches with Contra 4, so I suspect MM9 will be in the same boat: A love letter to fans. Capcom knows its audience and that its franchises have loyal followers, so I can't imagine the game not being just like everyone remembers. It's the way Mega Man should be.