Agreed on all counts. A lot of folks are sure they have the answer, usually something about a wet summer followed by cool nights in September. Like you say, it's probably impossible to account for all the variables. Cloudcover is a big one and it isn't always perfectly matched to precip anomalies, so it gets overlooked. Even things like rainfall chemistry and atmospheric pollutants affect plant behavior, so it stands to reason they probably impact leaf color in the fall. Then there's the subjective aspect. If the weather's nice and you're healthy, spending a lot of time outdoors and just generally enjoying yourself, that's when the leaves look their nicest, regardless of whether they're a couple shades less vivid than they could be.