Various responses to multiple posts:
More DR means less contrast - yet contrast could be considered more important in the eyes of many.
More dynamic range means more contrast, not less. It means that the photo can represent a wider range of brightness values. Yes, if you then deliberately squash the entire dynamic range of the camera into an sRGB image, you'll have less perceived contrast on the things that matter, but why would you do that? In a more realistic post-processing workflow, you'll end up with the same amount of contrast, just with bigger safety margins for correcting overexposed or underexposed images (which you can throw away disproportionately to make the picture brighter or darker) without blown out whites or blacks.
99.9% of the time, that doesn't matter. But when that .1% happens to burn you on a photo that you care about, the two-stop difference between a 6D and a crop body makes a world of difference, and an extra stop or two on top of that would be most welcome, even if it is mainly a "just in case" feature.
Folks want more MP, and yet on at least a few websites, reviewers note that without a tripod, there is no difference between 24 MP and 36 MP. And, yet, people want even more MPs!
There's no reason that this should be true in principle, so if that's what folks are seeing in practice, it probably indicates that the IS systems are simply not precise enough.
Unless you print larger than 8" x 10" there is virtually no difference between photos taken with an SL1 with kit lens and a 6D with "L" lens, but people want to believe that they need the best cameras and the best lenses.
Under optimal conditions, there's little difference between my 6D and my iPhone 5. Of course, real-world shooting isn't always optimal, and poor lighting, long distances from the subject, etc. can make those minor differences turn into huge differences.
The big reason folks want more megapixels, in my experience, is the ability to crop more in post processing. If a camera's resolution is good enough, you can shoot wider, resulting in fewer misses, confident that you can crop it in post and get a reasonable image.
This particularly comes into play when shooting wildlife and sports, because of the distances and the sometimes erratic subject motion characteristic to both. For both situations, a full-frame camera with the pixel density of a crop body would be a serious win. Unfortunately, Canon chooses to keep full-frame pixel density relatively low, presumably because they've been unable to scale the high-density sensors up to larger sizes while maintaining an acceptable reject rate. That's pretty annoying. Lots of us would really like the reach of a crop body, but without the crop.
On the plus side, I hv to admit Canon still has a clear lead in in-camera jpg rendering for portraits/skin tone.
With flash prices as cheap as they are, I stopped shooting anything but RAW several years back. From what I can tell, outside of a few specific markets (e.g. news gathering), that seems to mostly be a consumer feature. Unfortunately, consumers are buying fewer and fewer dedicated cameras these days, so I doubt that Canon's lead in in-camera JPEG rendering is very meaningful in the grand scheme of things.
I don't think the 5DIV will be a 'mirror slapper', or at least not if you don't want it to be.
With this talk of a 'modular' dslr going round I think the next 5D could have an interchangeable viewfinder, that is an optical pentaprism one for those that want to shoot stills with a high quality OVF, and an electronic EVF for those that want to lock the mirror up and use it for either stills or primarily video.
This way you get the best of both worlds.
Although that could certainly be done, it would make a lot more sense to just use a hybrid viewfinder that can be an EVF or an OVF on demand. And a properly done hybrid design could also allow for all sorts of overlays even in OVF mode (e.g. zebra stripes on overexposed areas, focus peaking, etc.)