There’s the crash generating tone into an existing track with the Mixer Board open, and the drawing of vertical green lines in the audio tracks when manipulating a slider in the Mixer Window, neither of which has made it to bugzilla yet.
Audacity’s lack of control automation is a shortcoming IMO, but may be too “DAW-like” (we don’t want to become Ardour) and not something the Team wants to implement. Note that when I say “automation” I am using the term as a recording engineer - click a button beside a fader that says “record automation”, play the project and adjust the fader: the software “records” the fader moves and can then later “play them back”. This is similar to the current envelope tool, but dynamic. In this way one can build up a mix, adjusting individual tracks, and tweaking levels until it is just right. “Automation” as used in the wiki and by the devs refers to something I would call “scripting”.
Unfortunately implementing this would mean changing the way envelopes are defined. They currently appear to be log or exponential curves defined by two points. Recording volume fader automation usually requires writing many volume control points with linear interpolation between them, and then possibly smoothing the curve (and removing redundant points) after the fact.
This is a long-winded way of explaining why I think the Mixer Board is not particularly useful. Since there is no possibility of recording the fader moves, the mix is necessarily static. One can raise or lower the level of a track for the entire length of the song using the volume faders, but you can’t “mix” it in the sense that I am used to, e.g. bringing up the guitar for the solo and bringing it back down under the vocals. Yes, you can do it while the project is playing, but you can’t “Mix and Render” what you’ve done.
So I am not surprised that users are confused as to its purpose. It merely replicates the functionality of the Track Panels (volume, pan, solo and mute) in a way that mimics a mixing desk. Users of hardware mixing desks will know what it represents. And DAW users (like me) will look at it and say “where is the automation? What is the purpose of a Mix Window without automation?”. The per-channel meters are useful, but could be added to the Track Panels in the project window.
I note that “Umixit, powered by Audacity” has 2 users. 
I also note that Umixit.com is “under construction”. My impression is that there is virtually nothing out there to purchase that can be used in this way. For the CakeWalk version you can’t download songs (only purchase them on special CDs) and they have some kind of DRM that restricts how you can use and share your mixes http://www.cakewalk.com/owners/umixit/faq.asp
I am still waiting for a defence of the Karaoke window. 
– Bill