The big difference is that it isn't just POSIX compatible, it is a full out "bug-for-bug" emulation of the Linux kernel specifically with an eye towards binary compatibility. Out of the box, the userland here isn't just GNU, it is entirely the same binaries as shipped by Canonical in Ubuntu.

Old Interix required retargeting and recompiling binaries, similar to how you would need to recompile to target the differences between Linux and BSD. WSL runs Linux binaries without the need to retarget or recompile specifically for Windows.

This also means that the long tail ecosystem of Ubuntu PPAs and custom binary download sites all mostly work, too.