So is there a definite "ratio" here that we're talking about? And is it time specific? I.e, in the 80's Coupe's had HUGE greenhouses, so is a 2012 E-Class 4-Door a Coupe and an '87 560 SEC a Sedan, because the new 4-Door E has a much slimmer greenhouse?
Also, would that mean that a Hyundai Sonata is a Coupe? It has a very slim greenhouse. How about a Chrysler 300 or Chevy Malibu? Both have very slim greenhouses, probably slimmer than some new Coupes (for example, slimmer than a new CL's greenhouse, more sheetmetal to greenhouse ratio, I'd think).
Again, I really think there are too many variables. It can't mean simply frameless windows, as a 5 GT would be a Coupe, and so would an old Subaru Legacy. It can't mean bucket seats in the back, or else a Lincoln Navigator would be a Coupe. It can't mean a sloping roofline, or else a Chevy Malibu or Hyundai Sonata (or anything else coming out these days) would be a Coupe. It can't simply mean limited headroom up front, or else a Lexus ES would be a Coupe (dismal frontal headroom), and a 6-Series 2-Door or Porsche 911 wouldn't be a Coupe.
Could it mean all those factors put together? That would mean that the CLS redefined "Coupe", as it bears all those factors. As much as Daimler Marketing would like me to believe that, I don't, and I think that would mean that we're rewriting the rules.
To me, it HAS to be 2-Doors to be a Coupe. Greenhouse to window ratio is far too variable and would mean too many practical Sedans coming out nowadays are Coupes.
I disagree with both of those (prefer the E and 5), because I like a more stately and commanding Sedan rather than a slope-y one (I like slope-y Coupes), but yes, they are in essence more Sedan really. The CLS, for example, the new one, doesn't even look so Coupey anymore. It looks much more Sedan than the admittedly very dramatically "I want to look like a Coupe" original CLS.