There's a discussion on the Lua mailing list on why OOP should/shouldn't be in the core.

It's Déja Vu all over again... the same discussion has been raging for years in the Tcl community. Both languages are more general than just OOP. And both "suffer" from a perceived lack of it as a consequence.

In my opinion, OOP is not all it's cranked up to be. When you're creating new DSL layers or doing the kind of meta-programming Lisp is famous for, then forcing everything into OO turns it into a straight-jacket - limiting ways in which to think about abstraction. Or to put it differently: yes, OO is great to model real-world objects, but that doesn't carry over to DSL design and some forms of abstraction/decomposition.

(full disclosure: I've been going through SICP again recently, and the meta-circular evaluator and Y-combinator - fantastic stuff)