Everything is a string.
introspection
event-based programming
simplicity
slave interpreters
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!...