This is perhaps the most frustrating sort of writing known to humanity, and it's not just a case of tl;dr (I've recently worked out what that means, and I've been dying for a chance to use it) although that's a part.
OCD says I ought to read it first, since it's the introduction and all, but when I choose a book, I usually want to read it, not read about reading it. I suppose it's meant to be a sort of tasting notes, telling you what to look for if you want to be highbrow about it and not just enjoy the book, but quite apart from the tedium I'd really rather read it myself, intact and undissected.
This is but a minor irritation next to the spoilers. How often do you find a succinct synopsis, robbing you permanently of every bit of the author's painstakingly crafted suspense and mystery, should you be so incautious as to read the introduction as an introduction?
Perhaps even more infuriating is when the writer of the introduction confines themself to describing
The most important event in the story.
It's just not fair to put that sort of thing at the start. It ought to be at the end of the book; an extroduction, if I may be allowed to coin such an absurd-sounding word.