Excellent, Phaz. We definitely needed a topic on this.
Kelsier mentions that Allomantic power is latent before you Snapped. But, since he's the guy who told us that there's always another secret, I don't think he is the most objective on the subject.
There are interesting things to say about Kelsier, though. I would think getting originally caught by the Lord Ruler would be pretty traumatic. Instead, his powers waited until Mare died. Maybe that is because Mare essentially kept him going until she died, when he Snapped. Still, it seems that he could've Snapped many times during his time as a thief.
I think the trauma of the Snap is in direct relation with the powers you'll have. Just thinking from my Chemistry class today, which was about state changes. There's a thing called Heat of Vaporization (or Enthalpy of Vaporization), the energy it takes to move something from a liquid to a gas. Gases are very energetic compared to a liquid, so Vaporization energy is always very high for gases. Now, the Vaporization energy differs for different substances. A substance like salt (which has a boiling and melting point because the ionic bonds in them are very powerful) will have a far higher Vaporization energy than something like water.
So say for example, we have a "Trauma point" that is proportional to the amount of power you have as an Allomancer. More power requires more trauma.
Therefore, some noblemen mistings who were less powerful would need less traumatic experiences. I basically just restated what you just said, but with some nonsensical cross-referencing with chemistry.
Let's talk about Vin, too. Kelsier says there could've been many times Vin could've Snapped. Well, since she is so powerful, under this theory it would take a lot to get her to Snap. So, what point would she have Snapped? I think when her mother slays her sister. That seems traumatic enough.