Well, don't worry, this game is definitely going to be much more difficult than some of the later Zelda games.
Let's begin with enemies. You have to swing your sword a certain way to kill many enemies, this adds a tougher element to the game. It may require multiple slices just to figure out where to attack to do any damage. I also heard around Zelda Dungeon in November that all enemies do at least one heart of damage. The ability to do that can easily kill Link, so some people may get anxious and just swing the Wii remote around wildly. This could open Link up to many attacks, and he end up dying quicker than a person using precise sword slashes. And some bosses such as Ghirahim seem to be very powerful. Ghirahim is able to grab Link's sword for goodness sake! And then some enemies are puzzles too. You have to figure out where to hit them and with what. Between everyday enemies and bosses, Link is going to have trouble with just them, at least until you have the controls mastered.
And then there are dungeons. We've seen a few puzzles, and I think some seem a bit daunting. But we also need to think about the whole returning to dungeons. We may not know which dungeon to revisit, Fi may decide not to give you a hint of any sort. It may even be a bit difficult simply to figure out which is a dungeon and which is the overworld; Miyamoto has said they seem to blend more and more together in this game.
You also need to think about the meters. The stamina and shield meter definitely limit Link. Unlike in Twilight Princess, he can't just raise his shield and then be safe forever. The shield is going to break after a certain amount of hits - and the more powerful ones can more than likely break the weaker shields in a single hit! The stamina meter is very important, it's what allows you to sprint, climb, and possibly many other type of physical activities. I can imagine sprinting up a wall will require a lot of stamina. This Assassin's Creed-type move would require a lot of physical conditioning and ability. All you can really do is hope that we will get a stamina meter upgrade as we continue.
There are many other difficult elements to the game, but these are a few. Skyward Sword is definitely not going to disappoint people looking for a challenge.