Changes since Lua 5.1

incomplete!
Here are the main changes in Lua since its last release. The reference manual lists the incompatibilities that had to be introduced.

Language

new lexical environments.
hex escapes in strings.
tables and strings honor the __len metamethod.
Libraries

new function package.searchpath.
new metamethods __pairs and __ipairs.
arguments for function called through xpcall.
Implementation

emergency garbage collector (core forces a full collection when allocation fails).
ephemeron tables (tables with weak keys only visit value when key is accessible)
handling of non-string error messages.
udata with finalizers are kept in a separated list for the GC. relevant?