In this case, "something" appears to be the full-array LED backlighting that's crucial to picture quality. While Vizio is still opting for backlighting that's easier to control than the super-slim edge-lit sets offered by other manufacturers, the D-Series uses half as many lights as the M-Series. If you can see them for yourself then take a look at how well they handle light bleeding around bright text on a dark background or dimly-lit movie scenes. After that you may get an idea of why more light sources helps for better contrast and black levels, and in the M-Series, it does an excellent job for the price.
Still, a $600 price floor is hard to beat and Vizio as usual is packing in the features. The TVs have 5 HDMI ports, plus 802.11ac WiFi that connects built-in apps including Netflix, Hulu, Spotify and Plex. You should remember that another reason for that connection is because Vizio's business model involves scanning content you might be watching to pull information for advertisers -- so go ahead and disable that "Smart Interactivity" feature (instructions can be found here) unless you want your habits mined for its InScape scheme.
Looking at TVs that are more expensive could also add support for features that are missing here, like HDR. Vizio is calling its version of the color-enhancing technology Dolby Vision, and on videos that support it -- like the new Ultra HD Blu-ray discs that are about to come out, or streams from Netflix and Amazon -- it can do more to improve your picture quality than just jumping from 1080p to 4K. The only problem is that, again, you won't find that in a $600 Ultra HD TV, so it's really a question of what features are important and how much you can afford.
These new D-Series sets are available now directly from Vizio or at Walmart (in-store and online). Of course, if you can't decide if you want one of these then just wait -- CES is only two weeks away and we'll be comparing them to new options from every other manufacturer.