Radagast wasn't 'invented' for the movie, but he was largely added too (He is mentioned but never seen in the books) and that rabbit bit was stupid, as was the death of the goblin king
I actually didn't mind the rabbits. For me, it boils down to, "Screw it; he's a wizard." He wants magic bunnies? He gets magic bunnies.
Though, it'd be badass if he had elk with massive, razor-sharp antlers. It could have made the chase scene a little more interesting.
I have to say that the death of the goblin king was really quite anticlimactic. The quip he made didn't really add that much. It was kind of humorous, but since that scene was a letdown anyway, the remark's humor lost its edge. I certainly wouldn't expect someone who could manage ruling over thousands of goblins to maneuver himself into that position.
I didn't feel it had to take up almost 3 hours, but I didn't regret spending that time watching the movie, except maybe for that chase scene. RunPee suggested this as a great time to run to the bathroom. I didn't have to go, but I kind of wish I did.