Fluidity

Fludity
http://www.decipher.org/archive/2006/01/ui_fluidity.html

Fluidity:
* transparency (don't make me think)
* responsiveness (don't make me wait)
* "gestural efficiency" (How efficiently I can complete common multi-gesture acts)
o distance betweeen gestures (literal and cognitive)
o necessary precision of each gesture
o interruptions

Finally, a perfectly fluid interface would feel more like dancing than working, which suggests that rhythm and balance play a role as well.