I skip introductions too. Even if they don't give away the ending, I feel like they tend to skew your reading of the book so it's in line with the introduction, even if that's not the only or best (or even a good) reading. Much better to read the introduction after when I have the information to judge.

I used to like explanatory footnotes, but as I get older, the more they seem to interrupt the pace of the story for me. Even if I don't flip to the back to read them, the footnote number is still there, nudging me.