[Everything is a string]. 

Wrong! 
Tcl has the data type, array, and in 8.5 the dictionary data type was introduced. Both of these are defiantly not strings. And, what about procs and namespaces? These are not strings either. People that say everything in Tcl is a string do not know what their talking about.

[JE] Actually, dictionaries are strings, just as much as lists are.  But you're right: there are plenty of important entities in Tcl that are not strings -- for example, open file channels, Tk widgets, variables (both scalar and array), encodings, and interpreters.  But in Tcl, all of those things are accessed by name, and the name ''is'' a string.  That's why, for example, you can't pass an array to a procedure (the array is not a string), you have to pass the array name instead (the array name 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."

----
[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 (a likely cause of bugs).  Likewise, lexical scoping can be implemented with namespaces ''when the programmer requires it''.  Again, a much better solution in most situations than enforcing it "never" or "always".
----
[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],
...