Features not performance
I just bought a Surface Book... I would have bough the Surface Pro 4, but the Surface Book looked so much cooler. I was leaving my old job and I didn't know if I'd manage to keep my Surface Pro 3, so I ordered a Surface Book for $3200 after all the taxes and shipping and absolutely love it.
It's basically just a laptop and I can virtualize a small data center on it while presenting. I've had 20 virtual machines and 20 routers and switches up and running on it. This is too much machine for almost any real purpose.
These days, thinner and lighter is no longer a feature... we're thin enough and light enough. Longer battery? They're up to 12 hours now. More powerful... 99% of everyone probably is more than happy with a Core i3 and 4 gigs... maybe 8 of RAM. Better graphics... maybe... I did buy Final Fantasy XIII just to have something to test my new laptop with... then I moved on... no real point. Better screen? 3000x2000 on a 13" screen allows me to make the text smaller than I can comfortably read and I have better eyesight than most... probably the only thing I have that works well.
I wouldn't mind :
A better pen, I carry a Wacom Cintiq with me because the Surface Pen (and iPad Pro Stylus) suck so bad they're useless.
Better support for iPhone headset volume control buttons.
Thunderbolt would be nice, though I'm not sure why.
An SD card port that is flush with the card so it doesn't dangle.
A second screen has always been on my wish list... but I can't see this being a major selling point.
Better wireless display support
A better power brick adapter... can't really say I like the Surface Pro one and USB-C tends to suck up one of my precious USB ports requiring me to carry a hub with me.
Make the entire PC detachable as a phone... in other words, find a way to fit the Core i7, 16gigs of RAM and 512GB SSD into a phone and make the Surface Book a screen, video card, battery and keyboard attachment.
Better touch pad... this one feels too much like the Apple one and I abandoned Apple because of their absolutely awful keyboard and touchpad.