Apart from the Tk and Expect extensions, one of the best parts of Tcl its general extensibility through the Tcl C API. Unfortunately, this is one of Tcl's less well-documented features. Most books and online tutorials either ignore it completely or, at best, provide perfunctory discussion and examples.

The other poorly documented feature is the objected-programming framework which was belatedly added after Tcl fell out of the mainstream of programming languages and so languishes unused.

The biggest problem of all is that that there is no online forum for Tcl users now that Google has dropped Groups. Usenet comp.lang.tcl was the main discussion forum and it is now gone.