In the case of DKC on the SNES, it had a proper challenge, but it got on my nerves because you could only save progress every three-to-four levels, and running out of lives meant you had to go back and replay several levels to get back to where you kept dying. I didn't mind the difficulty, but I hated feeling like I had to replay levels I had beaten already when I just wanted to hurry up and get back to the one I wanted to be on. I'm not too big on save scumming, but I'll abuse it for that, where I can just load a save state at the start of each level instead of having to waste time going back each time.
Really though, I think the VC adds the "golden banana" or "white tanooki suit" or "invincible Arwing" that these older games need. They are meant to be challenging, like others have said, and I would never want that taken from them. But they do need EZ Modes for those who don't want that level of challenge and just want to progress through the game. Now they sorta have it.