I have used Lisp, Scheme, Prolog (and multiple dialects of each) for extended periods in large projects. Each was a pleasure to work with. They all have their own advantages. I am not sure if there is value in arguing over x feature of one language vs. y of another. As a matter of fact, it was fun to write Prolog interpreters in Lisp, and Lisp interpreters in Prolog, for example. (Many courses / textbooks (used to) present these as side projects to do and the task is simpler than it sounds.) One could say Tcl is really an extension of Lisp or Prolog in its core concepts of syntax, data and program equality, style of interpretation, etc.

I agree wholeheartedly with the last paragraph of Mr. Ousterhout's reply here - and I must say, a smart, classy and almost Tcl-ish way of a jab at its distractors:

http://vanderburg.org/old_pages/Tcl/war/0009.html

I came across Tcl/Tk while doing Motif in a large C-based project. I simply couldn't believe how powerful, simple and succint Tk was compared to Motif (or anything else since). And it ran on Windows, Unix, Linux and Mac OS, to boot. It was too late for that project to switch, but all my other projects have used Tk if they ever needed a UI.

Similarly with Tcl. I still think the best introduction is Mr. Ousterhout's original Tcl/Tk book. It stands as one of the best language books on my shelf. Combined with Tk, one can put together a working application prototype in no time.

Of course, this was eons ago. Nowadays, Tcl offers one of the best environments with a complete set of packages ranging from web servers to image and sound processing. Plus, you can distirbute your program and all of its media and support files easily as well.

I believe Tcl has been mischaracterized and has suffered in terms of open popularity. But for insiders, it remains as one of those secret indispensable Ninja tools that is used over and over again for competitive advantage.