> upvar and it's interesting syntax allowed to create new "statements" for example in earlier versions of tcl (because looks like it was added later) you could implement try-catch "statement" and it would behave as if it was part of the language.

This is one of the things I miss most when using other languages. Outside of actual lisps (of which I consider Tcl to be one), there's very few languages that allow you to build your own control structures. Ruby comes close, but doesn't really have the same flexibility.