I'm not sure that Lua is going to appeal so strongly to Python programmers. It's wordy and its design makes opposite choices from Python's; vide global vs. local, for example. I think the brevity and safe assumptions of Python are a lot of what people value in it.
Thats if you only focus on syntactic issues like that. If you see how the languages work they are actually much more similar (both have generators, operator overloading, lexical scope, etc...)
Using any syntax, "local unless you specify global" and "global unless you specify local" are different in an important way. And syntax is important to most programmers; there are dozens of languages that provided the semantic equivalent of Python before it existed, but haven't matched its popularity. Python's practical syntax is much of the reason.
