cons:
- 1-based indexing
- you basically always want a local but you have to ask for one, it's a bit noisy. 'strict mode' is easily applied to the global environment, and catches most outright errors here.
- there is an adequate amount of library and extension code, but less than you'd expect coming from Ruby or Python. The lack of a blessed object system sometimes means taking more effort to understand what other people's code does.
pros:
- variadic functions and multiple return values make for a truly dynamic language
- first class functions which are fast and work well: everything is a closure, a whole file is just an anonymous function called a chunk
- the table as a single compound data structure works very well, and metatables are a unique and excellent solution to extending behavior. Array portions are required to be compact (non-nil from 1 to #tab), which can be worked around on the few occasions when it's inappropriate.
- asymmetric coroutines.
- first class environments, which are just, yep, tables. this allows for a few really nice things.
- fast. LuaJIT is very fast. Both Lua runtimes fit in a typical L1 cache, and it uses a register VM, which is a good fit for real processors vs. stacks. No global interpreter lock, all C level Lua functions receive a Lua_state as the first argument and are fully reentrant.
- in general the emphasis on minimalism means the whole language "fits in head", and the tools provided are more than adequate to get jobs done.
- the LuaJIT FFI, which parses C declarations directly, is simply a pleasure to use, and is the only one of which I'm aware where you pay an absolutely minimal penalty to use it. There's just nothing like it. More languages need to have this.
miscellaneous:
Semicolon insertion is simply a non-issue, unlike JS it was implemented properly. I have well under one semicolon per file, less than one per project, even. It's very, very occasionally nice to write more than one statement per line; that's what the semicolon does.
It's a nice language.