Purpose: identify a set of features that someone writing extensions would like to see included in Tcl (and Tk). See also Application Developer's Wishlist for Tcl and Application User's Wishlist for Tcl.
One guideline to help determine whether the feature is an urgent one or not would be whether or not the feature is currently available via an add-on or not. If available via an add on, then, in my mind, the urgency is lowered.
Urgent features needed in Tcl
Needed (or useful), but not urgent, features needed in Tcl