While still not out yet, we're learning a little more about what Windows 10 S, the imminent version of Windows 10 that'll run Store apps but nothing more, will and won't be able to do.
First, a thing 10 S won't do: run command-line applications. CMD and PowerShell, the two built-in Windows command-line interfaces, won't be supported. Neither will the Windows Subsystem for Linux (WSL) that allows the use of Linux software on Windows. The rationale is that the built-in command-line applications include dangerous tools (for example, the diskpart partitioning program) that can break things, and the Store has no third-party command-line tools at all. To keep Windows 10 S protected against user error, they're all prohibited.
Oddly, at Microsoft's Windows 10 S launch event, I was successfully able to run both CMD and PowerShell on a number of the Windows 10 S machines that were on display. Although the obvious ways of launching these things were removed (no entry in the Start menu or the Win-X menu, for example), the programs themselves did run. Leaked builds of Windows 10 S do appear to properly prevent their execution.
The Linux limitation has surprised some people, as Microsoft will soon be distributing Ubuntu, SUSE, and Fedora environments for WSL through the Store. This does not, however, make WSL a "store app," because WSL itself is not distributed through the store.
For most people, upgrading from Windows 10 S to Windows 10 Pro, and hence removing the Store-only restriction, will cost $49. But there will be some exceptions. The first one we already knew; anyone buying the new Surface Laptop will be able to upgrade to Pro for free if they do so by year-end.
The new exception is similar to one that already exists. The free upgrade from Windows 7 and Windows 8.1 to Windows 10 officially only ran for the first year of Windows 10's availability, with one exception: anyone using assistive technology—screen readers, braille displays, that kind of thing—could use the free upgrade in perpetuity. That same group has the same exception for upgrading 10 S to 10 Pro. The process of obtaining this free upgrade will be disclosed later in the year. Third-party screen readers and other assistive technologies have to integrate into the operating system at a lower level than is permitted for Store applications, thereby creating the need to upgrade to Pro.
The next major update to Windows 10, the Fall Creators Update will also have a number of accessibility improvements built-in. These include automatic generation of image descriptions, system-wide color filters to help people with color blindness distinguish between colors, and, for developers, the ability to disable the display and use only screen narration to better test navigation and application accessibility.