I have commented on this a few times just these past few days, but I must do it again here. As I pointed out before, all gamers can play an easy game, but not all gamers can play a hard one. If you make a game harder, it slams the door shut on less skilled gamers. Moreover, a harder game is not necessarily fun. Mjbd said, "There is a thin line between challenging and tedious," which is almost exactly what I have been saying, though my structuring is more of, "There is a fine line between enjoyable challenge and frustration." It is almost the same, but I wanted to use my phrasiology because I feel it to be more accurate. I want to point out that frustration is never, has never been, and will never be fun, and a game that is not fun is a worthless game.
Believe it or not, as I have used frequently as an example, my girlfriend declared Wind Waker to be too frustratingly difficult and vowed never to touch it ever again. As for how easy or hard a game is varies from one beholder to another. Mario Sunshine is a more difficult game, regardless of how many people here might line up to post arguments to the contrary. The problem is not how easy games are, or are getting, as was suggested the other day, but that gamers are getting better. What was considered challenging ten years ago is considered simple, these days; the reason being that gamers are more skilled now than they were back then. You can't simply pump a game full of challenge, though, to accomodate (spelling?) the really skilled because you shut out all of those that aren't. An easy but fun game is a thousand times better than a difficult but frustrating game.