It's not hard to sing if you're a guy. It sits on the A for a while, which is (I'm told) in the chest range of a girl, and then it goes up to c's and such, which are typically head range notes, unless you're belting, which isn't good for the song.
For guys, all that stuff is in the comfortable chest range, and I say this as a bass.
Blame Jason Robert Brown (who is, I might add, a guy), who wrote it.
I don't think it's hard at all, I learned it in two days oddly enough. I was surprised with it myself but when I did find that listening to the recording of it twice helped me hear better when I went off.
I found learning the piano accompaniment quite a bit more difficult that learning the vocal line. I played and sang it for a coffee house at my high school.
The piano acccompaniment is hard. Many a time did I get a cramp in my fingers trying to master that. I just kept saying "Jason Robert Brown was not going to conquer me!" It helped slightly.