I searched for a while and didn't see anything about this so I feel safe enough to post :P A feature I would like to see added to the dynamics of water is capillary action, at least horizontally, so a one pixel wide path that zigzags (height variation only 1 pixel) is a viable path for water, instead of it having to be flat or only going in a downward direction.
Capillary action, is the rise of liquids between two surfaces. So a straw of diamond could allow liquids to rise between the two layers through adhesion.
EDIT: Lockheed's got it, the pressure required to do this currently is a little too much for intricate stuff. currently only the top path in the save will work, and the bottom path doesnt. If capillary action were implemented (a width of 1 pxl would be good, then the bottom path (or one similar) would be fine.
So basically what your wanting is a form of osmosis? If so, That would be a very interesting addition. Some gases already do that. It got annoying making my "mixture" creation because all the gases went nuts! they just passed through my diamond sphere