Version 9 of How Tcl is special

simplicity

slave interpreters

temporal arithmetic: Among the Tcl testimonials is Tony Summerfelt's that, "if i need a program that does any kind of heavy duty date manipulation I use tcl. The clock command is easily my favorite in tcl."

Jacob Levy 2003-06-05 Methinks Tcl has the best and simplest thread abstraction available.

KJN 2004-11-29 upvar, uplevel, and the fact that language constructs have the same syntax as function calls.

You can write your own language constructs if you don't like the ones supplied with Tcl
You can use upvar if you intend to access a variable in the calling scope; in some languages this is impossible; in others, dynamic scoping makes 'all' such variables accessible

Also see "idiom", "What is the advantage of Tcl over Perl", "Beginning Tcl", "Tcl warts", "Arts and Crafts of Tcl-Tk Programming", "Is white space significant in Tcl", "Tcl Heritage", "What is Tcl", "Tcl and other languages", Is Tcl different!, "Why to use Tcl" [L1 ], ...