This is tough. Obviously, as others have said, "Objects in Space" isn't really an ending. However, and I know this isn't the popular opinion, "Serenity" really isn't that good of a movie. I know, I know, it's Joss and it's Firefly so people love it, but if you look at it critically, there's a lot wrong with it. The jump between the second and third act makes virtually no sense. As @Evil UU says, it tries to be both a reboot and a continuation, which muddles things up. Again as others have mentioned, the deaths of Wash and Book feel like Joss jumping up and down saying "it's MY movie", which almost distracts from the scenes that are meant to mean so much.
Don't get me wrong, it's not "bad", I do like "Serenity", but it could have been a LOT better, there are huge holes in it, and I feel like because Firefly got such an unfair ending and because it's Joss, people give it a pass. I hope there's more to the series some day, just to give it a real ending.