I think it has always been, at a very basic level, the sheer sound of it for one, and then the amazing organization and layout of it in terms of the theoretical aspects of music. From there, the actual feel of it, the topography itself and the simplicity of black and white, groups of threes and twos, repeating patterns, the range that it covers. And then, of course, the huge body of repertoire written for it. The instrument lends itself to orchestral imagination, and for me it lends itself to imagination in general, yet it can be "about" extremely focused and simply tasks, too . Musically it is amazingly versatile, it can sing, it can be a single voice, it can be a choir, it can be an orchestra or a particular instrument, it can sound legato, staccato, it can stand alone in a performance or perfectly support any other instrument, including an entire orchestra. The more you get to know it, it seems the more there is to get to know. The mechanics of it are incredible and beautiful to me.
You could sleep in it or under it, you could attach a motor and use it as a vehicle .... It's just plain great !