I don't think that's a completely fair assessment. It's true that the language has some gratuitous feeling warts (not being zero based is a very severe flaw where embedding into C/C++ apps is a primary use case; it also messes up luajit and terra's UX no end, global by default is another problem, albeit one that can be mitigated with tooling). But what other language packs so much punch for so little weight? Despite being quite simple, the syntax still is expressive enough for a lot of DSL uses. Lua has proper tail recursion and an elegant co-routine design and it's slightly idiosyncratic lpeg library is worlds better than the regexp offerings of most languages. It has serious meta-programming power based on a much simpler model than e.g. python or ruby.

Despite its warts lua still feels like a better (imperative, statement-based) scheme than scheme to me. What scheme offers a similar combination of speed and expressiveness for the same conceptual and file-size weight as lua(jit)?

Wrt to repl: the torch7 repl is quite serviceable, so it's worth giving it a try even if you're not into deep learning.