It's not so much things that are left out or glossed over, though there are some of those; there's a lot about the way wizarding culture really works that seems inconsistent. Some of it is plot holes -- there are some big ones in book two, though at the moment I'd have to go and look them up in my notes, and some even bigger ones in book five.

And I'm okay with those being Rowling's mistakes -- as you say, this is her first series. But they're still there, and they don't make for particularly enjoyable reading, a lot of the time. :)