By the time you calibrate it UHD is just a lot brighter. Unless its a projector size the 4k UHD details are slightly sharper and brighter.
This may be the case now, but there seems to be a big enough push from the movie studios (with content), all the way through the TV manufacturers to make UHD more than that. If that is all that UHD is, then nobody will really care. If we can get high dynamic range and better and expanded colors, then that will make the whole thing very special. Just think about all the rave about the Pioneer Kuro plasmas and the last generation Panasonic plasmas. It was all about how great excellent contrast ratio and black levels look in a picture. The same is true on the projector end, how the JVC projectors are all the rave for the same reason. These displays and projectors have good performance in all areas of the board, but what sets them apart from all of the competitors is the contrast ratio and black levels. To a somewhat lesser extent, even when we consider the OLEDs by LG people get excited by the picture, even though they have had issues with uniformity of color and from what I remember with the first generation issues with shadow detail. It seems to be that contrast ratio is likely the biggest factor that can improve the perceived quality of an image. The exciting thing about high dynamic range is that we can get improvements in local contrast to levels we have never seem before. That is what gets me excited. This and all of the other stuff in the UHD package is enough to make the jump from HD maybe as big as the jump from standard definition to HD was, even without the extra resolution.
That is not to say that the industry can't just mess up the whole thing. Like just cranking the brightness and altering the expanded colors so that everything is artificially vibrant, to try to sell more TVs in the showroom with these more high performance TVs and saying that this is what UHD can offer. Just letting the good TVs show the extra performance that they have in UHD with quality material should be enough to sell the whole concept.