IronPython interoperates with a whole host of C and C++ code. I'm not sure why this would matter?

The initial implementation may need to assume single-threaded C interface support and take a global lock but it wouldn't be a stretch to have these things declare they are multithread aware and relax that restriction.

Forgive me but most of these objections seem like post-hoc rationalizations. The first step is deciding to support a GIL-less multithreaded mode. After that, solve the problems one step at a time.

It is amazing how many times accomplishing "magic" boils down to:

1. Decide we're going to solve this problem. 2. Iterate toward the solution in manageable steps.

#1 is by far the most difficult aspect :)