I agree that Boost is not on the top of the list of things this distro should care about. It was just the first example that came to mind which breaks the ABI with *every* release (if not in reality, the libraries get renamed for each version which makes it a moot point).
Yes, first, whatever is required to implement freedesktop.org standards, hardware, and external communications should be included (starting from the bottom with a kernel, systemd, udev, dbus, upower, udisks, *dm, pulseaudio, cups, firewall, etc.). After that, get the user-facing applications done in an upstream-oriented way (app storeish). After that, I, at least, would like to see common libraries be provided by the system which apps can assume exist. The set of libraries included could be part of the interface declared for a version.
The reasoning is that I would think that development should still be easy, so getting libraries and such provided by the distro should be possible instead of going around and downloading umpteen dependencies to start some project. Unless a "developer's" store makes sense, but I think distro packages do better than an app store model would. Of course, that is further down the road after user applications work.