There are fundamental design choices in Lua that can certainly put some people off, such as prototype-based inheritance or the union of arrays and maps.
It certainly wasn't casually hacked together, though. Ierusalimschy et. al. have thoroughly documented the language and its development rationale with admirable clarity.
In particular, the paper "C APIs in extension and extensible languages"[2] will be of special interest to prospective embeddable language developers. "Passing a Language through the Eye of a Needle: How the embeddability of Lua impacted its design" is also a good read, for those who can summon bits through the ACM hellm... er, portal.