Ross Douthat of the Atlantic online (and greatest last name ever) explains why the last Harry Potter was pretty lame. He argues (and I agree) that Rowling took a lot of cop-outs, and hinted at complexities that might have made it a more interesting book.
In my personal opinion, Harry should have bought it when Voldemort killed him, not some stupid almost-death. Neville should have then killed Nagini AND Voldemort, in a nice ironic twist, showing that it was the prophesy that made Harry great, and that it could just as easily have been about Neville. After all, isn't the whole point that Harry isn't the greatest wizard? Just brave with good friends?