I've found myself really disappointed with this book, more so than with any other book in the Potter series. I read the whole thing in less than 24 hours, so obviously I found it engaging ... but there are a number of details and plot points that I felt were sloppy at best (and lazy at worst).
For example, one of my friends and I have been talking about the "Trace" on underage wizards. In Book 6, one of the characters states that the Ministry can only detect where magic is being done, not by whom, and that it's up to parents of wizarding households to enforce the no-magic ban. Then, by Book 7, suddenly there's a "trace" on underage wizard magic? Why was Harry blamed for Dobby levitating the cake, then? Introducing that concept throws inconsistencies into several key parts of the overall storyline, and frankly, it wasn't really necessary, except to introduce an epic battle early on in the plot.
Then there's the whole Harry-as-Horcrux bit. What seems fuzzy to me is that if Harry only survived Voldemort's
Avada Kedavra because his mother's protection lived on in Voldemort, wouldn't that protection end as soon as Voldemort died? Theoretically, even if Harry could survive based on that protection, shouldn't he have died once Voldemort died? The concept was underdeveloped, and the explanation Dumbledore gives Harry in "King's Cross" is patently vague and unsatisfying. And frankly, I think it's a cheap plot trick--either kill Harry or don't kill Harry, but don't resurrect him from the dead.
And speaking of raising Harry from the dead, I was disappointed that Rowling ultimately proved unimaginative enough to resort to a classic Judeo-Christian Messiah storyline in order to resolve the plot. (Did any one else catch all the religous allusions throughout the book--the verses from Matthew and I Corinthians on the Dumbledore and Potter tombstones? Lee Jordan's radio handle, "River," as in the Jordan River? The hilt of Griffyndor's sword appearing as a "silver cross" in the lake?) I mean, really, when the series ends with the resurrected protagonist declaring that his death has protected all the people he died for, and he tells the antagonist that his "last chance" of a happy afterlife is to repent of his wrongdoing, the entire story becomes semi-allegorical in retrospect. I've always thought the books explored ethics and the nature of good and evil from a decidedly secular perspective, so the religous undertones of the last book were a complete surprise to me. I really thought Rowling was going to be more imaginative (and realistic) than to suggest that the defeat of a single "evil" character would make the world a better place. The end (especially the epilogue) rang hollow after the relative moral complexity of the earlier books.
I could go on, but this post is long enough already.