Changes in the Language

The vararg system changed from the pseudo-argument arg with a table with the extra arguments to the vararg expression. (See compile-time option LUA_COMPAT_VARARG in luaconf.h.)
There was a subtle change in the scope of the implicit variables of the for statement and for the repeat statement.
The long string/long comment syntax ([[string]]) does not allow nesting. You can use the new syntax ([[string]]) in these cases. (See compile-time option LUA_COMPAT_LSTR in luaconf.h.)