[Everything is a string].

[unknown]

[trace]

[send]

[introspection]

[the idea of scripting]

[event-oriented programming]

simplicity

slave [interp]reters

The [pipeline]

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."

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

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[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
----
[KJN] 2004-12-01 the ability to replace Tcl commands with user-defined functions.  The use of [puts] by the interpreter for ''all'' Tcl output on stdout and stderr (not just for output where the programmer has used a [puts] statement).  Therefore, if [puts] is replaced with a user-defined function, that function captures all output, even error messages.
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[KJN] 2004-12-01 delivery of Tcl and Tk as shared libraries by default.  Even tclsh and wish use these shared libraries.  Embedding the interpreter in an application is therefore the norm.
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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" [http://phaseit.net/claird/comp.lang.tcl/tcl-why.html],
...