A really good, properly scanned 35mm slide film resolves at a level of around 10-12 megapixels. The level of detail from 36 megapixels is on par with traditional medium format film, which means a 35mm lenses weaknesses will be more much more apparent with the D800. Ironically enough though, the pixel pitch of the D800 sensor is just slightly larger than that of the D7000, which means the D800 is no more demanding of lenses then the D7000 is at a pixel for pixel level, at least on a general scale. Obviously things like corner weakness will come into play with full frame, but if you take the center 5 megapixels on the D7000 and the center 5 megapixels on the D800, those pixels on the D800 will not test the resolving power of a lens any more than the D7000 would. The D7000, just like any of the newer/higher megapixel cameras like the Canon 7D, Sony NEX7, A77, etc, all test a photographers shooting technique and a lenses qualities just as much as the D800 will. The only difference is this time we are seeing this test played out on a Nikon full frame camera, so corner performance of current lenses is really the only area where new insights as far as a lens quality goes, will come from. If the center of a say a 70-200 is good on a D7000, it will be equally good on the D800. ...and of course if your shooting technique is sloppy, it will show, just as it will with any of the other high megapixel cropped cameras.
I do agree, the D800 will not make you a better photographer, and most people may never utilize all 36 megapixels. But for those who are demanding of their equipment, and the D800 is designed to be a high end professional imaging tool after all, the D800 is a ground shake. For instance, 14.4 stops of dynamic range compared to 11.5 on the D200 is a massive difference that equates to a photographer having to use a 3 stop grad filter for a landscape shot and dealing with those limitations, or simply not having to. It means dealing with a lot more blown out highlights or lost shadows in one city shot, or seeing a more natural complete range of detail and tones. In terms of colour reproduction, 25.3 bits compared to 22.3, means more life like, more flattering, less jarring transitions in colour, from skies to skin. 36 megapixels is the difference between having a sharp, nicely detailed 24x36" print, and one that looks soft or has jagged edges. And lastly, the high ISO performance between the two equates to more than 7 stops of usable light. That's like a wedding photographer shooting with a F1.8 lens on one camera and a F9 lens on another. Will any of this stuff make a difference to someone who can't utilize it, of course not, does it make a difference to someone who can, you bet.