I was looking at SF Signal's Harry Potter Outreach Program and I thought that in several ways, the appeal of Harry Potter is that it absorbs a huge age bracket (I mean try giving a kid copy of Ursula le Guin's The Dispossessed, he'll regret the experience). Of course the same could be said of most children's books or even young adult novels but obviously, they aren't as big as Harry Potter.
However, I think one advantage the series has is the fact that they're being read today, when it's fresh from the printers. I mean J.R.R. Tolkien didn't hit it big until years after the initial release of Lord of the Rings. Harry Potter ain't a cult following, it has a following, period. And I think that gives Rowling some freedom which other authors haven't been able to tap.
Look at all the other children's books or young adult novels. The language and style and tone is all consistent. In the Chronicles of Narnia, the tone of The Lion, the Witch, and the Wardrobe is the same as Prince Caspian, even if the latter book takes place further in the series. Same goes for The Dark is Rising series and His Dark Materials series. Most likely, the reader who reads book 1 will be able to read book 3 (and some people read it in reverse order).
I don't think that's the case, however, with Harry Potter. Give a kid book 1 and the same kid won't necessarily be able to tackle book 6 (for adults it's not a problem). Back in college, I was telling my teacher that since the format of the Harry Potter books is that each book takes place in one school year, perhaps the same reading formula should be followed. And in many ways, we've been instinctively been reading it that way because we're current and we're following the scheduled releases, which is spaced more than one year apart. When book seven gets released this week, it marks the end of the series, and people in the future who've never read the books will read them in a variety of ways, everything from chronologically to out of order to even randomly picking a book in the series to check if it's any good. And while the Harry Potter books are quite standalone (it has a definite beginning and end), there are various elements that I think benefits the reader if they read it in a chronological fashion (but that's a typical problem of serial books or books that belong in a series).
Moving back to Harry Potter, the present readers grew and waited for the release of the books. Kids who read book one when they were ten will probably be reading book seven when they're eighteen. That's why the book transitions don't suffer. Again, I must stress the fact that the upcoming problem doesn't apply to adults or even young adults but to children. The later books pack a punch, at least in page count if not in content. So a child of say, age seven, can't read book six--at least read it and understand most of what's going on unless that seven-year-old is really really smart and has a lot of life experiences. The "knowledge gap" between books is small (say from book one to book two and book two to book three) but even the smallest gaps widen in the face of a a seven-book serial that took almost a decade to write. And Rowling ain't C.S. Lewis--her writing and characters and topics mature as the series progresses (which isn't a bad thing mind you). And I think that makes Harry Potter unique (of course me being not well-read, I'm not sure if there's any children's books out there that becomes more mature and complex as the series progresses to the point that you have to be a "young adult" reader when the series's ends).
Having said that, I'll probably go with the advice of Guy Gavriel Kay when it comes to reading children's books (and I'm liberally paraphrasing here): "As a parent, you should know what your child can read and cannot read. Don't let labels like "children's books" or "young adult" limit what you and your kid can or cannot read. If he's ready to tackle the classics or novels, let him (or her) do so." And I think that sums up any rating system, whether it be for movies or video games or whatever. Those rating systems are just guides and the maturity and growth of each individual varies from person to person. If you take a personal interest in the growth of your child, you'll know whether something is appropriate for them or not, instead of simply relying on ratings.