The penultimate Harry Potter story is something of a reprieve, a year of things being mostly back to normal. This is admittedly contrived, and the Voldemort-only-attacks-at-the-end-of-the-school-year trope is most egregious here.
However, if we accept this contrivance, this volume could be considered to be the consummate Harry Potter novel. We are able to enjoy the Wizard School motif again, really for the last time. There are moments where book 6 recaptures the wonder of book 1, as our heroes enter their advanced studies. The romantic relationships get more attention here than they do in the rest of the series, which further orients this particular volume as a boarding school story. (I find that Harry/Ginny works for me and Ron/Hermione doesn't, although I'm sure I was biased by Rowling's recent statement about the latter.)
The largest flaw to me is that the real action happens mostly offstage and has to be retroactively explained in the next book. While Rowling always left little mysteries to be explained in later books, nothing else in the series is retconned the way the ending of this book will be. Basically I feel that she went a bit too far in keeping Snape ambiguous until the last possible moment. The result is that there is an element of falsehood in the emotional arc of this story, which prevents it from really standing alone.