Yes, Tcl has it's problems but it seems a shame to see it appear to approach end-of-life,especially with the recent 8.6 release - although it is only in 8.6 that an 'official' object system has been added so maybe that says something.
Obviously Tcl has Tk which I think was for a long time a significant attraction - certainly was for me in the 1990's. The ability to script, cross platform GUI applications that glued bits of existing c code was exciting at the time. I had a significant amount of image processing stuff working this way with some very workable Tcl/Tk interfaces using OpenGL. That was fun too.
However, I've been using Lua for some of the same reasons that I used to use Tcl. In particular I like the simple nature of the language especially allowing a multi-paradigm approach to development. Although I have heard some argue it just supports many paradigms equally badly on single person projects, it's expressive enough. It's code base is small and neat - Tcl was always kept in good shape - and it is obviously designed for extendibility but with just enough of a core language to be of value on the vast number of platforms on which it runs. There are a reasonable number of addons and libraries available but the small core is a primary characteristic of Lua. Thus to get greater functionality you need to 'craft' your own setup which allows you to deploy just what you need for the task - the language is uber dynamic in that respect. LuaJIT is a nice bonus and the performance figures certainly suggest some stunning improvements are possible - not used it myself. Whilst understandable, it is a shame that LuaJIT isn't totally compatible with 5.2. Lua even has a pretty close approximation to Tk in IUP [http://www.tecgraf.puc-rio.br/iup/].