Lua is semantically more similar to Scheme, according to the language designers. The resemblance is quite intentional. Ease of embedding into C is a first-priority of the language implementation. It's almost trivial to interface to C, and in many cases it's easier to write bindings to the Lua API than to massage SWIG generated interfaces.

"userdata" are first-class values in Lua. They are containers for pointers to arbitrary C structures, and by overloading their behavior it's extremely easy to interface between C and Lua.

Lua tables are extremely elegant, and are the basis of object and class systems, all data structures, namespaces, packaging, and libraries.

The Lua interpreter is quite fast by itself, and with the LuaJIT written by Mike Pall your speeds can be even greater. The rumor is that the first public release of LuaJIT 2.0 is coming this winter... that one is a tracing compiler based on the same concepts as Tamarin, SpiderMonkey, and V8.

Yes. It seems to have many of the same abilities as Scheme (and I forgot to mention it has coroutines.), though it doesn't have read / the sexp syntax or macros. It's extraordinarily portable, though, and also seems to also contain most of what I like about Python (though it lacks its remarkable libary).

I read The Implementation of Lua 5.0 (www.tecgraf.puc-rio.br/~lhf/ftp/doc/jucs05.pdf, 15ish page pdf) last week, experimented some with _Programming in Lua_ 2nd ed. since, and I've been very impressed with how tasteful the overall language design seems. It seems like a brilliant tiny language, much like Forth and its descendants. (I've read that the Lua source is good reading, too. See: http://www.reddit.com/comments/63hth/ask_reddit_which_oss_co...)

The Lua Emacs mode is nothing special, but I'll see if I can improve it. :)