I'm hoping that the Hobbit parts will keep the relatively light-hearted tone, and that the connective bits (which I'm assuming will be in the third movie) will sort of bridge the two. Since no one dies in The Hobbit (apart from, you know, bad guys) and literally thousands of people die LOTR (although mostly nameless faceless thousands, apart from the two kings and Boromir), I think you need something to gently ease people from the one into the other. It's like the difference between Sorceror's Stone -- all happy magic where only bad guys die (and even that off-stage, as it were, since Harry passes out) and Deathly Hallows, where we lose George and Hagrid and Moody and Tonks and Lupin and and and and... Quite the bloodbath, so it would be a jolt to go straight from the one to the other!