It's expected that there will always be naysayers, people who hate because they can hate and it's trendy to do so. In my experience, Rowling is the best writer I have ever read. She stomps Tolkein, who is a fan of loading the story down with so much description that no one cares anymore, she even tops King, who has had a large inspirational effect on my own writing. The ending was, in my opinion, flawless. Voldemort's end was not brought about by superior strength, which Harry has never had in either magic or brute force. He was not beaten by sheer luck. He was beaten by his own ignorance, which has been a paramount principal through the entire series.
As for the Prologue, I believe it was put there more for closure than anything; there are huge bases of fans that have been obsessed for some time over the various relationships in the books. Harry/Ginny, Ron/Hermione, etc. It answers those questions rather decisively, and as Templar mentioned, keeps writers from doing anything but really writing about Harry as an old man. Some of you might think this a rather assholish thing to do, but as both a RolePlayer and an aspiring writer, I sympathize with the urge to keep that which she spent so long creating as her own, unchanged by others.
Yes, it does leave open the possibility of sequels, especially coupled with the fact that Harry simply left the Resurrection Stone in the midst of the Death Eaters. Rowling, however, has explicitly stated she won't be writing anymore. Even if she did write about Harry's or Ron's kids, I wouldn't begrudge her a bit. I sympathize, as well, with the desire to keep the story going. It's a saddening thing, to permanently close off something you've put so much time and heart into. I wouldn't expect her to be doing it for the money. Check sales figures on the books, the movies, the merchandise, all of it. She gets a good cut of that, I sincerely doubt she wants more badly enough to spend more years on writing when she could easily retire.
So, off the subject of ruthlessly bashing Rowling. Who wants to place bets on how long before a new author picks up the story with a new school?