The electrostatic sound, to me, is a certain "sheen" to the music, like a slight lump in the high frequency response. Not sure at what frequency it peaks, but I think it's just a little bit lower than the presence region.
And, like tubes, it doesn't exist in all electrostatics. Just as you can have a very tubey tube amp (Wheatfield), or a very dry tube amp (Singlepower [mine, anyway -- not all of them]), you can have an electrostatic rig that has that signature sound, but you can -- through tuning -- get rid of it as well.