''Purpose: enumerate what makes '''T'''he '''c'''ool '''l'''anguage special'' -- [Richard Suchenwirth]
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   '''Simplicity''':   The [man] page Tcl(n) [http://www.tcl.tk/man/tcl8.5/TclCmd/Tcl.htm] teaches you, in 11 rules and 264 lines, just about everything about the syntax: basically it's about semicolon, newline, whitespace, braces, brackets, dollarsign, pound/hash (*). Of course there are "words", but all on the same level as the [proc]s you write. List elements are joined with whitespace, this allows for beautiful human-readable coding with little effort (see [Salt and Sugar]). Bidirectional [pipe]s are available with just
     set p [open |foo r+]
   '''Unity of data and code''':   after all, just about everything is still a string [http://wiki.tcl.tk/IsAString]. If you know (or want to find out) what you're doing, just [eval] it.
   '''Introspection capabilities''':   see what vars, commands, procs, [widget]s, ... you have available; for a proc, dump its args and body; ..., let widgets say their configuration... "Supraspection": Look above you in the call stack with ''[upvar]'' and ''[uplevel]''
   '''Lazy parsing''':   No processing step subjects code or data to more parsing that absolutely necessary. This allows commands that completely disagree about the interpretation of e.g. * (such as [expr], [regexp], [scan], and [glob]) to coexist happily, and more generally makes it possible to employ [little language]s.
   '''Automatic conversion''':   surf between string, list, integer representations with no explicit conversions (**). This may at times cost performance, but at other times allows beautiful simple code, e.g. add a list of numbers with 
     expr [join $list +]
   '''Internationalization''':   [Unicode] (see [Unicode and UTF-8]), and thus just about every widely-used writing system in the world, is allowed in all strings. How much needed and how fully implemented this is, is sometimes debated. The [read] and [source] commands can't handle Unicode files opaquely [["transparently""?] yet. But with all its flaws, Tcl offers the best i18n environment I've seen so far. [Tk]'s mechanism of finding a Unicode char in other fonts is extremely helpful when you go global. See [The Lish family] for helpers from 7-bit ASCII to 16-bit Unicodes.
   '''User-friendliness''':   Even most Tcl error messages are kind of a joy to read - endless loops (recursions) aren't endless anymore but detected and reported. Most commands are implemented in a pretty common-sense, evident way. Quite a number of Tcl friends read, and often answer questions, in news:comp.lang.tcl
   '''Openness''':   you can do much more with this extremely flexible language than you'd expect, by combining string manipulation, eval, and sometimes the unique '[unknown]' command (see [Radical Language Modification]). Consider Stephen Uhler's [HTML] parser in 10 lines. You can write Tcl procs that are called in [SQL] syntax (but never the other way round ;-) [[what does this last mean?  [CL] believes [Postgres] prooves the contrary.]]
   '''Clarity''':   [Perl], in contrast, is hard to understand.  For instance, the Perl man pages contain 493 occurrences of the word "magic" - used when the details are too overwhelming to fully explain - it's easier to just say "the right choice will be made, it's magic, trust us".  Even the Programming Perl book uses words like magic, incantations, etc, all over, which is startling considering that it's supposed to be a reference.  In contrast, the Tcl documentation contains zero uses of magic or anything remotely suggesting something is going on behind the curtains that is simply too hard to grasp. The Perl source echoes this inability to explain the details and contains over 400 occurrences of the word "magic.  The Tcl source contains none and in fact is beautifully documented.

   '''Tk''' is the best, easiest [GUI] scripting toolset around.:   Proof is that the Perl and [Python] colleagues also use it, even if they have to emit some Tcl code for that. But the freshest Tk is always bundled with Tcl. 

   '''[Expect]''' is the best, easiest tool around for automating, testing or [gluing] interactive programs.:   Even Perl, Python, and [Java] experts prefer Expect over trying to do the same thing in other languages.
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* Couldn't write this up there :-)
 ;\n\s{}[]$"#

** I replaced the word "shimmering" with "automatic conversion".  
Shimmering is not only meaningless to most people (and therefore smells of 
intentional showing off), but the word refers to unfortunate frequent conversion, 
which is the exact opposite of the point you're trying to convey.  
"automate conversion" is a lot more pedestrian but clearer and ultimately better all around.  
(God forbid people should understand what we are trying to tell them.)

For fun and serious code goodies, see [Bag of Algorithms] - [Bag of Tk algorithms] - 
[Braintwisters] - [Tcl Gems]!
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HD: Another point: there are obfuscated [C] contests and obfuscated Perl contests, 
and half the activity in cplm seems to be about posting incredibly obscure one-lines 
that print out "just another perl hacker", but when's the last time someone boasted 
of their obfuscated Tcl code?
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Why on the [Obfuscation] page, of course!
'''DGP'''
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[RS]: Just yesterday I glanced at an older GC[LISP] manual - they have more than two dozen types to choose from. Or [Java]'s class hierarchy, where it takes many minutes just to see what's all there. With Tcl it's different: you don't have a monster toolbox to wonder at, just knife, fork, spoon - and yumm!, off you go! ;-) 
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[RS], one more thought: Programming languages of old had arithmetics and I/O operations 
as central elements. Came C and said, "I/O is not part of the language, 
use libraries for that" (and only had to allow varargs for printf). 
Some years later came Tcl and said, "Arithmetics is not part of our language either, 
use specialized commands, e.g. [expr]", leading to a substantial simplification again. 
Q: can this trend be continued - are there parts in Tcl that may be defined out 
(without loss of expressivity)? Or is Tcl essentially essential?  
CL replies:  the cheap answer is, yes, of course, $-dereferencing 
is purely epiphenomenal [[deref ref]].
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Where can "Stephen Uhler's HTML parser in 10 lines" be found?

    ############################################
    # Turn HTML into TCL commands
    #   html    A string containing an html document
    #   cmd	A command to run for each html tag found
    #   start	The name of the dummy html start/stop tags

    proc HMparse_html {html {cmd HMtest_parse} {start hmstart}} {
	regsub -all \{ $html {\&ob;} html
	regsub -all \} $html {\&cb;} html
	set w " \t\r\n"	;# white space
	proc HMcl x {return "\[$x\]"}
	set exp <(/?)([HMcl ^$w>]+)[HMcl $w]*([HMcl ^>]*)>
	set sub "\}\n$cmd {\\2} {\\1} {\\3} \{"
	regsub -all $exp $html $sub html
	eval "$cmd {$start} {} {} \{ $html \}"
	eval "$cmd {$start} / {} {}"
   }

I dug it up here [http://freegis.org/cgi-bin/viewcvs.cgi/grass51/lib/form/html_library.tcl?rev=1.1&content-type=text/vnd.viewcvs-markup] [CMcC]
(''[escargo] 26 Jul 2005'' - I received ''Access to "grass51" is forbidden.'' when I tried
to access that link.)

[EKB] This is classy. I wanted to do some test runs to it and I think it can be simplified 
even further, so I made a page for it at [Stephen Uhler's HTML parser in 10 lines].
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See also [Tcl heritage] - [Arts and crafts of Tcl-Tk programming] - [Tcl/Tk is too easy]