There are some neat tricks you can do with the 2 stereo signals. Linear combinatios of L/R channels is the easiest, but doesn't work well with modern mixes where bass/snare/voice is centered and other instruments are rarely ever hard-panned (you can get really cool results on some older albums like Rubber Soul - there's a lot of hard panning going on there)
If you find the right mix of David Bowie's Space Oddity (one with hard panning), playing one channel makes it sound like THE worst garageband Bowie cover ever.
The high hat is high on the mix, you can barely hear the orchestra, and only one of Bowie's audio tracks is on it, so it is really discordant.
There are of cuorse more advanced techniques for isolation of centered tracks and vocals in particular see http://virtualdub.org/blog/pivot/entry.php?id=102 and http://dsp.stackexchange.com/questions/1499/how-to-extract-v...