The 12-40/2.8 is not a fast lens. For instance the Panasonic 25/1.4 was faster by two stops, but a wide aperture is only one part of the recipe to gorgeous bokeh. The other part is focus distance, or to be precise, the relation between focus distance and distance to the out-of-focus background. The nearer you get, the easier it is to throw the background out of focus. Ever wondered why you have so much trouble getting usable DOF while shooting macro? Well, that’s why.
Normally non-macro lenses are either good or they focus near. Traditional lens design tends to always trade near focus for lack of distortion, but this lens is an exception. It focuses almost down to the front element, and it does so in all focal lengths. Thus it is easy to get deliciously creamy bokeh even at 12 mm. I’ve always liked that in my Sigma 20/1.8 and 28/1.8 for Nikon, and I am certain to like it here.