The final battle sucked. They made it look as if the death of Nagini was the end of Voldemort, or that Harry defeated him with no effort.
Complete fail.
PLUS, no one witnessed it, so who's to say he's even dead?
To be fair, Harry did pretty much defeat Voldemort with ease in the book, and it all had to do with the fact that Voldemort was hell bent on killing Harry himself because of a prophecy he never truly understood (ie he didn't really have to kill Harry). When Dumbledore learned that he cleverly went about making sure that the Elder wand would be useless to Voldemort (by willingly allowing himself to be disarmed by Draco and then killed by Snape), and when the final battle came Voldemort could do no harm to Harry with the Elder wand because Voldemort was not its master, and the fact that the wands allegiance was actually to Harry it wouldn't even hurt him let alone kill him.
But I otherwise agree. The final battle was changed and contrived for typical movie fare, and it was done rather poorly. I really think they screwed it up when they strayed from the book by having Voldemort chase Harry all over the castle in battle, when they really just should have stayed true to the book where we get a good sense of just how powerful Voldemort is when he takes on several powerful witches/wizards (including McGonagall and Kingsley) at once before Harry finally intervenes and is able to defeat Voldemort because the Elder wand is fundamentally useless against Harry.
I also thought the Harry v Vold sorta sucked. Maybe I expected better after the Order of Phoenix battle. Sad to say that the best wizard battle in all the movies occurred in that movie instead of the grand finale.
I didn't care for it either, but as I've said before its not like that fight was epic in the book either. Granted that didn't mean they couldn't have produced an epic battle by having Voldemort fight other wizards before Harry could confront him, which
did happen in the book (albeit those events weren't fleshed out at all in the book, only that Voldemort was holding his own and even beating several powerful wizards all at once)
Its a shame really, as there was a flash of brilliance in the "duel" between McGonagall and Snape in this movie that was reminiscent of Voldemort vs. Dumbledore but it was just too brief to be anything spectacular.