I CAN'T TELL YOU WHAT IT IS, BUT I KNOW IT WHEN I SEE IT
To understand the esthetics of a tiara, you must know what she does. First you see the tiara in relation to the person wearing it. So, if it is too tall, it looks like the person will fall over. If it is too small, it is not noticeable as a tiara, If it is too tall relative to its horizontal measurement or too wide relative to its height it looks wrong. Generally speaking, it should be about 1-1/2 to 3-1/2 inches tall and 1-3/4 to 2-1/4 its height wide. <
Next, the tiara has an outline. This should, within limits be straight from the side to the front and should not have sharp or abrupt changes. There can be bumps and depressions along the way and even VERTICES work if they follow one after the other in a proper fashion. What does not work are abrupt changes to this outline where it drops off or jumps up too sharply. It can start at a lower angle and get steeper averaging about 45 degrees but that kind of thing is within reasonable limits; you would not do well to have it start at 20 degrees and then about half way in, sharpen to 60. I have never seen one that starts at one angle, say 60 degrees and flattens out as it goes inward.
The shape of the ELEMENTS creates in the mind of the observer a kind of motion. That means, it starts somewhere and ends somewhere and there are points that stand out, usually these are the CENTER STONE or PRIME STONE, other stones or clusters of stones and the APEX. Butterfly Tiaras have no APEX in the center, instead there are two peaks elsewhere, this is disruptive. Another thing is the stone size. A crystal tiara does not use stones that are larger than an inch, or about 25 millimeters in any measurement. Larger clusters are all right but a single object in an environment where the norm is about 10 millimeters creates the disruptive phenomenon of gigantism and is unattractively bizarre. In short, you want the motion along the tracks of the ELEMENTs to be smooth.
A tiara is a whole that is greater than the sum of its parts. The additional aspects are either added psychologically by the viewer or come from the way the parts fit together to form the whole. Its value is in part determined by the user. If the tiara is for a special purpose then that is part of the value. The user's esthetic judgment also plays a part. 
s you go through the world of tiaras, you will find that you like some, are not moved either way by some and you hate some. Since tiaras are made for many purposes, including for one-time big doings, esthetics are often sacrificed to the calling attention to the doings. So the tiaras I am talking about are for ordinary use. Also some tiaras are de trop; too tall so that they look ungainly, too gaudy, but there seems to be a following for that (it just does not include me).