All threads would use the same ***instance*** of all globals, so if you save something in a global variable in one thread, it affects all threads. This is a very undesirable trait in server threads. If you forget to make a variable local and write to it you have a problem.
I hate to say it, but I think that this is a flaw with Lua. Most of the time you want to declare locals, and that should be the default. Compare the usage frequency of Lua's 'local' keyword to Python's 'global' keyword; the difference is very significant.
This then gives us the the problem of context, that is saving state from one subroutine call to another in a given thread. You can't use globals. In the past I've used per-thread tables that I pass to all subroutines, which works, but this idea of cloning threads sounds like it might be a more natural feeling solution to this problem.
