I've found quick development time to be my biggest gain from using Lua, and that stems from the language's overall good design. The authors of Lua have made smart choices in data structures (hash tables) and control structures (lexical closures / first class functions, coroutines.)
Lua uses hash tables as its only native data structure, giving it a slick, flexible implementation, and it works really well. Say I need to hang onto some collection of data. Whereas in a language like C++ I'd have to agonize about which type of data structure I want, in Lua the answer is always "a table," so I make it a table, it works, and I'm done with it. I love that. I think other languages like C# are starting to figure this out, abstracting over their major data structures as just "collections."
You can take all that with a grain of salt, as I have little to no experience in the major scripting language players (Perl, Python, Ruby) and couldn't tell you to what degree they do this as well. Maybe Perl, with its "hash" structures, is similar.
Anyways, it's just a really slick little language overall, I enjoy programming in it, and I get things done in it quickly. It's been good for scripts and small tools, and I'm experimenting with writing larger apps in it lately, which has been going pretty well so far.
Thanks for your interest in promoting Lua!