While fans of wxWindows, Qt, GTK+, and other GUI toolkits often deride Tk as "old-fashioned", a good case can be made that it's merely mature. Well into its second decade of successful application, Tk remains the most reliable cross-platform GUI toolkit, besting even Java's AWT in its effective coverage. Beginners also find it easier to start with Tk than any other system, with the possible exceptions of rather hermetic and platform-specific technologies, as well as members of the *Card family.
Why has Tk aged so gracefully? It enjoys several specific technical advantages, discussed below. At a higher level, it seems increasingly likely that Tk is better off for being object-based, but not object-oriented. [Explain. Todd Coram makes the point that OO limits tagging to clumsy static kinds based on RTTI and multiple subclassing--yuck. Maybe we can get Todd to write more.]