What quickly comes to mind -- PDAF and all that goes along with it; the advantages of optical VF such as no lag and no power draw; much longer battery life, just to name a few.
A smaller/lighter SL is certainly achievable, I'm curious why they went full DSLR size. And the 24-90 is a behemoth...
Interesting ... Given that I'm a fairly dedicated manual focus user, I don't know what goes along with PDAF other than quick, accurate focusing. And it's not been proven one way or another that the SL will have any better or worse behavior there to me so far.
Much longer battery life is a question mark too. My E-M1 nets a typical 600-800 exposures out of the same amp-hours that my D750 does for the same typical number of exposures; I don't know that there's really much different given quality design and good components. My D750's viewfinder is not very usable without the power on, of course whether it is or not is a moot question since the camera isn't usable with the power off either.
No viewfinder lag is the biggest thing that an SLR has over a modern EVF, and that remains to be seen how different it is with the SL vs the E-M1 EVF. The E-M1 EVF is already such that I often forget it is an EVF. But the dynamics of the viewfinder are definitely different: with an SLR if you are capturing a sequence, you get a split second, freeze frame image just slightly ahead of the subject's motion, where with any EVF that freeze frame is going to be, at best, slightly behind it. So that's a difference.
I'm not concerned with the size of the lens ... I'll likely never own it anyway as I don't like zooms.
But the worst part of using the Sony A7 was fitting any practical, faster, lens to it: the lens overwhelmed the body's ergonomics instantly. The SL body is about the same weight and size as the Leicaflex SL body, which is beautifully balanced with even long, heavy lenses like the 180/2.8 and 250/4. Surely there's a place in the world for larger cameras like this just as there is for more compact cameras. It's those ergonomics that make the Leica M-P a bit of a pain to work with using the longer lenses where the R8 and Leicaflex sing with longer lenses.
So ... we shall see how it all works out. The more I read in the SL instruction manual, the more impressed I am with the design. It really does seem very thoughtfully laid out, and even if my intended use is only going to use about 40-50% of everything it has. I'll likely sell my D750 after I get it.
G