It looks pretty interesting. Without too many spoilers — it’s in the trailer, after all — the doomsday is caused by a planet approaching the Earth, and we hear someone say it was hidden behind the Sun.
Now, I’ll say that’s not really possible. A planet falling in from deep space and approaching us close enough to harm us would be visible for decades, and since the Earth circles the Sun once per year there’s no way the Sun could hide it for very long — if the Sun were hiding it in April, for example, by November it would be high in the night sky and visible to everyone. At first I interpreted the line to mean it was orbiting the Sun on the opposite side of our orbit, but that doesn’t work either; a planet big enough to hurt us would have revealed itself through its gravitational influence on other planets long before now (Gerry and Sylvia Anderson did a movie in the 60s based on this called "Journey to the Far Side of the Sun", in fact).
But that doesn’t matter. What good science fiction does is take a concept and see how it affects things, and in this case I’ll take a rogue planet as the plot device. "Melancholia" looks like a lovely movie, and I imagine I’ll watch it when it’s finally released in November.
Incidentally, a different movie called "Another Earth" will be coming out in a couple of weeks, but it’s also about a planet that appears in the sky, and also seems to be thoughtful and interesting. The trailer is on YouTube.