I didn't think about this part, because originally I was only thinking about math. Thank you for giving specific examples, so I can test my intuition to them.

Uhm... I'd say that "Δ . B = 0" doesn't trigger my feeling of awesomeness, however important it may be. Even "E = m * c^2" doesn't, and that has a lot of applause light connected with it. They are just too simple; they feel like "a + b = c".

On the other hand, I realized that if I don't know what the equation means, I can't decide whether it is good enough. So the meaning is a part of the utility function, but mathematical elegance is another part (the feeling of "oh, this is really equal to that?" like when the pieces of puzzle suddenly fit together) -- and I want the equations that satisfy both criteria.

Squark's examples are already great enough and probably all I need, so if you had some specific examples in mind, please post them here, but otherwise I already have what I wanted.