Actually, as far as traffic, I have seen a study that indicated click-through rates on the SERPs could affect rankings. If for a certain search a site comes up in the #3 spot, for example, but draws an unusually high percentage of clicks, the search engine may move that result up higher. This is part of determining user intent - if the closest match according to the algorithm is not drawing it's share of clicks, the engine may determine it is not accurately figuring out searchers' intent and will try to adapt. This is even more true with Google, which can to a certain extent monitor user behavior after the user leaves the SERP.

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The best way to learn anything, is to question everything.