A slow interface. If that's the only problem, then I wouldn't care. I usually configure it once, after that I forget about the interface unless the device needs a reset and configure every few days. LOL. But if you are a WRT 1900AC user, then this is important.
Regarding opensource. As I already mentioned the netgear gpl code is based on openwrt, so it is actually already possible to easily create your own build.
I bet my butt, that you will see official openwrt support for it. Someone at myopenrouter already mentions openwrt support.
Looking at myopenrouter download links, I'm also sure you will see dd-wrt for it, all of the latest models are supported by dd-wrt.
The R7000 is one of the units dd-wrt devs recommend.
The HW of the R7500 is incredible and I'm looking forward to a SNB review I don't trust CNET reviews.
Netgear and Asus do a pretty good job lately with their hardware, I had units from both of them in the last 2 years and they have been working great after their firmwares stabilized. But the R7000 was already superior in terms of cooling and performance and I'm pretty sure the R7500 will be even better with the 1.4Ghz cooling and eSata and 2 usb 3 ports. Funny though that the R7500 model will probably beat their top model R8000