Lua is pretty minimalistic. There are few batteries included. The standard library is very basic. There is no native object system, though implementing one is trivial.

That said, I friggin love Lua for its flexibility and transparency. Once you grok its few basic mechanisms, everything else just falls in place. In contrast, understanding Ruby or Python is a lot more difficult since the languages themselves are a lot more complex.

In fact, if I were to teach programming, I would teach it using Lua. Implementing your own object system is increadibly instructive. Learning about coroutines and lambdas is, too. And Lua is still small enough to just read its source when in doubt. It's bytecode consists of just a few dozen instructions.

Honestly, Lua is a marvellous language and I just love to use it!