Thanks for the comprehensive reply; however, a quick search turned up a couple of small, portable and embeddable Lisp dialects (Hedgehog, PicoLisp, ECL) that all seem to target the same niche Lua is apparently designed for. Admittedly, Lua has probably been developed for a longer period of time, but small Lisp interpreters shouldn't be very complex, so I guess they are pretty stable.
Lua is probably going to be faster, but one wouldn't exactly use a scripting language for math heavy stuff etc either.
Lua has had a long time to slowly and deliberately evolve, informed by feedback from a gradually growing group of users.
It's kind of the opposite of Javascript, which went from 0 to release in a few weeks and had a bunch of design errors become permanent as a result. Lua and Javascript have a lot in common, and studying Lua may be a good way to see where Javascript is going in the long term.
Lua is significantly smaller than ECL, and a bit smaller than PicoLisp. According to sloccount, they weigh in at 432,818 (ECL), 15,018 (PicoLisp), and 14,309 (Lua).
Lua is probably going to be faster, but one wouldn't exactly use a scripting language for math heavy stuff etc either.