Harry Potter....Book vs Screenplay

I took my teenager to the opening of Harry Potter last night. WONDERFUL visual effects and great acting....good movie. Now having said that, I'd like to compare the book to the screenplay.

I DEVOUR the books...buy them opening day and read them that day. But how does one cut a 700+ page ms down to a 300 page screenplay? (And I thought I had it bad cutting 10K words! LOL) Granted, in writing a book we must put in setting and description--movies don't have to do that because it's shown on stage. But how does the writer decide what pertinent details to include in the screenplay? There were several points from the book that were omitted from the screenplay in the movie last night. Some of them tie into upcoming books, so I'm wondering how they'll explain certain plot points in upcoming movies. A total subplot wasn't even mentioned in the movie. Something that in book 5 ties up a main plot point. So, I'm wondering how they'll pull that off in two or three more movies.

And here's another thought I'm pondering....the Harry Potter books hit the bestseller list as soon as they hit the bookstores. The movies sell out, bringing in millions from the box office. Some people, like myself, read the books then watch the movies. Some people read the books, but don't watch the movies. Some don't read the books (what?! Not reading?! YIKES!) but watch the movies. This imaginary story from JK is filling a void in the market today. Or maybe because it was so fresh and unusual in the mainstream is why it's such a hit. I don't know, but I do know that JK has pulled me into her imagination for a while, shared her story with me, and I'm entertained. Fiction at its best. I could only aspire to pull off what she has.