I've seen it twice now...
1. yes, they did leave out a lot... but as others' have said, when you have a massive book and the studios won't allow anything longer than 2.5 hours, you have to cut anything that doesn't really move the plot along. Gone are the days of GWTW where studios are more concerned with ART, as opposed to making $$$. As for Grawp, he is semi-crucial, because he was part of what happened with the Centaurs closer to the end. And it all fits into the larger story-arc, maybe not necessarily within this particular film... but it does all fit.
2. Many folks liked the early ones for two primary reasons: they could fit more of the book (already mentioned), and that they were lighter and showed more of his time IN school, as opposed to furthering the plot with Voldemort. But as the books grew, both in size and darker in tone, they had to decide what to cut.
Rowling herself says she really likes this one and that it's the closest to it's book (despite the stuff cut out), and if SHE gives her blessing, then that's that. If they followed the book faithfully with #5, the movie would be a heckuva lot longer than 4 hours, I can pretty much guarantee that. With the increasing number of ADD/ADHD kids and adults, no one really wants to sit through that long of a movie. Even on TV, when they have a story that's more than a couple hours, they split it up and call it a miniseries.
They're doing these to make money, and they're being as realistic as possible, in terms of making the films.
For the record, I liked this one a lot. It's much darker than the first two... and they're really improving on the flow and effects as they go. two more movies... I think they'll do just fine. Think about it, for years now, we've had 20-something actors playing teenagers (on TV and such)... I think leaving the originals in to finish out the series will turn out just fine.
My 2 cents...