The problem isn't the actual runtime: Django was as long, but I never got bored or noticed that it was going on so long.
The problem with The Hobbit is that parts of it, especially the action scenes, are really boring to watch. They play like scaled-down versions of LotR encounters, with no attempt made to make them interesting or unique and set pieces that often we've all seen done better in other films. The film seems terrified of ever letting things be dark, which leaves the meeting with Gollum far less atmospheric than it ought to be and makes the escape from the goblins in the mountain (even with all the we-wish-we-were-Jackie-Chan business with ladders and boards and the humorous interludes that don't quite fit) far less visually interesting than the one described in the book (tense fighting down narrow, twisting passages illuminated only by glowing swords and the occasional goblin torch).
Also, the three consecutive openings were completely unnecessary. It would have been much better to open either with the first old Bilbo segment, fading from the illustration of young Bilbo to his actual face, or to open immediately with young Bilbo and the opening line from the book (which makes no sense in the film's context of Bilbo narrating a story he intends Frodo to read, because Frodo knows exactly what hobbits and hobbit holes are) and then play the Smaug flashback over the dwarves singing about it, with the full song instead of the out-of-place narration (the film immediately cuts to a different narrator, the first narrator never comes back, and enough people fell in love with that song and requested a longer version after seeing the first trailer to demonstrate that audiences would have sat for it).