One thing I find interesting about Lua is that it has tail-call elimination, coroutines isomorphic to one-shot continuations, proper lexical scoping, and a "feel" similar to a (cleaned-up, better thought-out) JavaScript. The language has an entirely intentional semantic resemblance to Scheme yet seems to orbit that black hole rather than get sucked in, to abuse the metaphor.

I suspect that one reason it can do this successfully is that the authors are not afraid to break backwards compatibility with each new major release of the language. The fact that it is intended to be embedded within an application which ships with the appropriate revision of the language helps greatly in this regard.