It looks like many of the changes focus on maintainability, which is a good thing for Lua to be working on. Constant variables and moving the string-to-number coercion out of the base language are good for avoiding some pitfalls. More debugging help and warnings are surely going to make it easier to write good-quality code. Lua is really fun to write and I’m glad that they’re not afraid to make breaking changes to improve it. Lua 5.1 (in the form of LuaJIT) is also a plenty capable language; unfortunately it’s stuck in a sort of Python 2 vs 3 situation.
I notice with great relief that this doesn’t mention the experimental misfeature they added in an earlier release candidate where setting a table key to nil does not remove the entry from the table. One of my favorite things about Lua is that tables can’t have nil in them, so you can’t have nonsense like “I put nothing in this list of three elements, and now it has four elements” like many other languages have.