Hmmm. Okay, so if I were an algorithm and wanted to predict whether a user would find a site low quality, and I just got better at deciding how to use the data I have for doing that, enough to spur my creators to tell webmasters, essentially, "Minimize your quality-signaling SEO - we don't need your backend signals as much as we used to - and start really focusing on your users" (which is the common theme in Google insiders' advice since Panda, and yes, it's the advice given before Panda, too, but not quite so adamantly, it seems), then probably the data I just got better at wasn't any specific on-page factors, but that surrounding user behavior. Right? I mean, what else would it be, but matching algorithm's decisions to user's decisive actions?
I don't do it very well yet, though. My decisions might be more sweeping in variable-quality UGC sites in which I assume all that glitters is pyrite.
And my confidence in my decisions might be overrated - for example, if I associate erroneous signals with quality, such as assuming all valuable sites are those with which one would entrust one's credit card number (after all, people will trust their neighbors' advice on how to deal with menopause, but wouldn't let their neighbors have their credit card numbers, and would trust drugstores to take their credit cards, but not to give honest, disinterested advice on hormone therapy.)
But generally, my focus would be on better interpreting the actions of users. And there is no SEO for that - only end results.
If memory serves, Google has actually warned that they can't share the details about Panda because it involves something that could be gamed. Yet it's not supposed to be something easily tweaked by webmasters. That leaves user behavior, which can be gamed, if with difficulty.
Is that reasonable to suppose?