There are a very few truly global entities there, most notably the current working directory and the environment variables. They're global because the underlying concept leaks through into C code you link in and subprocesses you launch, and changing that would be really quite nasty. The Tcl interfaces to those things are internally protected against multithreaded access, of course, but you can still get yourself into a mess that way.

The bit I really like about Tcl is how easy it makes asynchronous I/O across all its supported platforms. This doesn't sound significant to Linux users, but on Windows that's a very very big deal...