It seems strange no one in this thread has mentioned thermal limitations of such a thin and light design. True current CPUs are more energy efficient than those in the past but you are still going to get throttled when the machine is working hard from what I have read.
Also as you said, user replaceable battery, ssd and ram.
Thermals are indeed a good reason too. While it seems that the current MB Pro does a reasonable job with cooling the CPU, it is obvious that with better cooling you could drive the CPU harder. As the laptop is for me a mobile workstation, there is also the point that I would prefer a slightly bigger laptop that runs faster.
Which is why Apple should differentiate between the "air" and the "pro" line more. There would be a place for a 15" air, which is a slightly thinned version of the current 15" pro, and a true pro machine which sacrifices a bit of portability for features and performance.
My current dream machine would be roughly like the late 2015 MB Pro (that happens to be the machine I am typing this on), with at least a 16" screen (the bezels are plenty large on mine) and 1-2mm more thickness, allowing for good thermals and more serviceable parts. For a pro machine a battery, keyboard or ssd exchange needs to be doable by a skilled technician on-site.
Also as you said, user replaceable battery, ssd and ram.