Harry Potter goes through hell and loses several friends along the way, but in the end, he is able to defeat Voldemort through The Power of Love.
Well maybe. It's not like they didn't think he was dead the last time the "power of [Lily's] love" killed him. Nor is there any evidence that the society that caused the problem in the first place has changed at all. Not to mention that most of Voldemort's supporters are still alive and free, while Umbridge killed most of the Muggle-borns.
Those are all valid points, but I think that the epilogue was meant to show that everything turns out amazingly well, at least for Harry and his Nakama.
Mainly the epilogue was meant to make sure no one would pressure her to do sequels.
Rowling has stated in interviews that [[spoiler:there were changes to the Wizarding world following the defeat of Voldemort, helped by Harry and Ron taking jobs as aurors and Hermione first working to improve the lives of House Elves and then taking a job in Magical Law Enforcement. Kingsley as the new Minister apparently fixed stuff up as well, and since virtually everyone and thing that supported Voldemort was at the final battle, it would be safe to say that most of the Death Eaters were killed or captured. The book implied that a lot of Muggle-borns went into hiding (and some were saved by the heroes). And Rowling also stated that Voldemort was trapped in the limbo Harry visited and would be unable to come back as a ghost, so by all accounts he's gone.