Incompatibilities with Previous Versions

Lua 5.0 is a major release. There are several incompatibilities with its previous version, Lua 4.0.

Incompatibilities with version 4.0

The whole tag-method scheme was replaced by metatables.
Function calls written between parentheses result in exactly one value.
A function call as the last expression in a list constructor (like {a,b,f()}) has all its return values inserted in the list.
The precedence of or is smaller than the precedence of and.
in, false, and true are reserved words.
The old construction for k,v in t, where t is a table, is deprecated (although it is still supported). Use for k,v in pairs(t) instead.
When a literal string of the form [[...]] starts with a newline, this newline is ignored.
Upvalues in the form %var are obsolete; use external local variables instead.

Most library functions now are defined inside tables. There is a compatibility script (compat.lua) that redefines most of them as global names.
In the math library, angles are expressed in radians. With the compatibility script (compat.lua), functions still work in degrees.
The call function is deprecated. Use f(unpack(tab)) instead of call(f, tab) for unprotected calls, or the new pcall function for protected calls.
dofile does not handle errors, but simply propagates them.
dostring is deprecated. Use loadstring instead.
The read option *w is obsolete.
The format option %n$ is obsolete.

lua_open does not have a stack size as its argument (stacks are dynamic).
lua_pushuserdata is deprecated. Use lua_newuserdata or lua_pushlightuserdata instead.