My absolute favorite thing about Tcl is its simplicity. Just do "man Tcl" (it's built in to Mac OS X). That one man page describes the entire language and it's only 12 paragraphs long. There's a wonderful simplicity and symmetry that comes from the core "everything is a string" tenet.

That said, I haven't used it for a new project in over 10 years. I still kind of miss it though...