At the SMC 2010 conference I had the opportunity to encounter a reacTable (a tabletop music interface) for real, with an introduction and some tuition by its creator Sergi Jorda. I remember being happy for them when Bjork took a reacTable out on tour, this new music technology getting an airing in front of many thousands of people and generating media interest etc - but at the time, I didn't find the youtube clips particularly inspiring, the reacTable seemed to be being used as a kind of glorified effects unit and slider controller. It wasn't really clear whether the reacTable was an interaction paradigm of its own or just a pretty way to control pd.
Under the hood, the reacTable is indeed controlling a patcher language for audio/music, i.e. something that will be familiar to many electronic music makers. But the basic interaction design is the key. For example, the reacTable is deliberately made circular, so that there's no "privileged position" and small groups of people can interact together on an equal footing.
The screen is physically not small (about 1m across maybe) but the blocks that you put on it have their own physical size and can't be "minimised" or duplicated like virtual things can - so the interaction is restricted to quite small graphs with a small set of generator/FX units. This limitation constrains it to be more of an instrument-like experience than an unbounded field of electric possibility - not a bad thing, a difference that creates different results, and ensures the system keeps to a manageable level of complexity.
A colleague of mine (Robin Fencott) is studying group interaction in these kind of situations, looking at public/private distinctions and whether people feel willing to modify "each other's" work on a collaborative interface. So I was interested to see how these issues played out on a reacTable, and there are a couple of subtle aspects that bear upon this. For example, the synth/FX blocks on the table are quite "promiscuous" - by default they simply connect to whichever suitable unit is nearest, meaning that if another block comes near, they may instantly disconnect from whatever they were previously linked with and pair with that instead. This starts to break down the distinction between different people's work on the table, since it's easy to accidentally connect your blocks with someone else's, and the physical limitations of the space (already mentioned) make this quite likely. On the other hand, though, different sub-graphs are assigned different colours, mainly to enhance the visual understanding of what's going on, but that does create a slight hint of "mine-vs-yours" when mine and yours are different colours.
Another important aspect of the physicality is that it provides for physical moves similar to what DJs do (scratching, cross-fading, etc). I don't mean the reacTable could be used for scratching - I mean I can imagine reacTable performers evolving a repertoire of physical gestures which make the most of the connection between their hands and the blocks - like DJs did once they discovered that a vinyl mixer could be used for more intricate stuff than simply crossfading every four minutes.
This potential was shown really well in a performance called KVSwalk_SOLO by Juan Parra Cancino, who proved pretty definitively that the reacTable is more than just a glorified pd GUI. He did a solo performance (so it didn't make use of the multi-player potential, but never mind) where he showed off some really deft gestures as he added, pushed, pulled and juggled the blocks around the reacTable surface, and made a great structured piece of music as a result. That for me was the decider - I won't say that Juan Parra is a virtuoso on the reacTable but he convinced me that such a thing is possible.