The DF to DF+ was the first product launch I've ever seen in medium format which I thought was under-hyped.
The new features are not that big a deal, an autofocus-adjustment option similar to the options in Nikon and Canon bodies to enter an offset to tune the AF of your body in case it is front or back focusing. This is a nice feature, but in five years it's been very rare for me to find a customer whose body had a bias, and when it did a fast turn-around tune up at the service center solved it.
The improvement to Autofocus speed, accuracy however were pretty big improvements in my experience. There were improvements to autofocus every generation from AFD1/2 > AF > DF > DF+. The focus locks faster, more confidentially, and produces more consistent results on long-throw-changes (e.g. going from near to far focus in a single shot). I use the DF+ at weddings and even in dim situations find it focuses more than adequately for that work. Focus TRACKING is still pretty poor, so you won't be shooting any sports with this body (or any other in MF as far as I've experienced) or otherwise tracking fast moving motion. But for anything that is static, or moving slowly/moderately/predictably you can get great results without hassle.
The DF in contrast had a habit of coming up a wee bit short of focus on low-throw changes, leading to a required technique many DF shooters dubbed the "double tap" where you would tap the focus once to get in the right ball park and another time to snap the focus into perfection. This isn't a huge problem for some styles of shooting, but clearly was not desirable. The DF has a feature option to change between "accuracy" and "speed" focusing, and the accuracy mode would mostly eliminate the above described annoyance but at the cost of additional time while the focus would tweak it's way into correctness. With the DF+ I've never found a need to switch to "accuracy" mode as I find "speed" mode is more than accurate enough and is, indeed, speedy!
The other improvement of the DF+ was simply to ask the service department which components of the DF were most commonly responsible for repairs, and beefing those elements up or tweaking the way they were used. The idea being to make the DF+ even more durable and reliable in long term heavy pro use.
