Realistically, any statement about difficulty is only going to be a rough guide. As people have said, players differ in skill level, but even entirely aside from skill level, one can find a game harder or easier based on playstyle or thought pattern. For example, my mind might work in such a way that certain types of puzzles are incredibly easy for me to figure out, while others are almost impossible. Even when it comes down to the controls and gameplay itself the same is true, as playstyle might make an otherwise easy section of the game incredibly hard specifically for that one person, or vice versa.
Take for example myself. I found The Minish Cap, a very easy game, to be pretty challenging because I'm an aggressive player. I like charging in and messing things up.
TMC is not designed in such a way to reward that kind of playstyle, and in fact punishes it pretty severely. The Wind Waker and Twilight Princess on the other hand often allow it to be rewarding, and even harder games like Ocarina of Time or Spirit Tracks. It's not always a matter of skill.
So gauging difficulty for other people is always going to be an approximation, and you can get a feel for its accuracy as it applies to you better if you go over how easy that person felt games you have played were and if they had about the same amount of trouble with it as you did, or see if there's a consensus about the difficulty and figure out if your skill level is average, or above or below average. These are things that can help you gauge the accuracy.
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More on-topic, though, I think that Skyward Sword is going to have above-average difficulty. I do not think it will surpass the 2D games, like Adventure of Link, A Link to the Past or the GameBoy Color Zeldas in difficulty, but I do think it will surpass or at least match some of the hardest of the 3D Zeldas (Majora's Mask, Spirit Tracks). It seems like a
lot of people feel it's a hard game, so I think it's safe to say with a fair amount of certainty that the consensus is that it's a hard game. I think it'll be a fairly challenging and rewarding experience.