Thor: The Dark World hinges on a last-minute twist that will have Marvel fans talking for hours. After Malekith has been defeated and the day is saved, Thor returns to his home-world to tell his father, Odin, that he can't take the place of King, and will lead a more fulfilling and productive life if he is allowed to go free to protect the other realms as needed. Odin eventually accepts that his son is better off living this way, and accepts the decision.
And then comes the big reveal, as Thor exists the chamber: Thor was talking to Loki all along. Yes, that crafty son of a God disguised himself in Odin's form and tricked Thor into allowing him to take up the throne as King of Asgard. The movie literally ends with a giddy Loki sitting atop his father's throne in the one position he's always yearned for more than any other. Which is to say, it's not made entirely clear what happened to Odin, only that Loki has either murdered him or put him somewhere where nobody will ever find him. Loki's rule of Asgard will, of course, make for a highly interesting plot point come Thor 3 (or before then, perhaps) - what will Thor do when he realises his brother isn't dead and has probably done away with their father?
Whether Odin is dead or not presumably hinges on whether or not Anthony Hopkins chooses to return to the series by the time the next sequel swings around.