If I needed an embedded scripting language for a project I'd reach for Lua without a second thought.
I've been strongly tempted to use it for actual core app development but every time I look at it the scarcity of common libraries compared to something like Ruby or Python cools me off.
It would be great if one could use Lua as general purpose language. The missing part is the lack of a bigger official standard library. Several addon libraries that try to fill the gap are outdated, unmaintained and/or not of the same high quality code as Lua and it's official libraries. The Lua community would also benefit from an website that has unified documentation and comment functionality like php.net and Ruby Gems or Node.js NPN repository functionality. (I know Lua has a Wiki and repository ...but sadly, they look very dated, its features are from last century).
Using Lua as embedded language is great though, especially with LuaJIT2 it is top notch. Interfacing with C is easy. C++ requires a bit of research as there are several possibilities (e.g. third party wrappers).
I've been strongly tempted to use it for actual core app development but every time I look at it the scarcity of common libraries compared to something like Ruby or Python cools me off.