Line in the sand: The point where I actually try to emulate anything paleo on the basis of my belief that it's paleo.
Paleo is a heuristic. You use it to generate hypotheses and to figure out what to eat for dinner. Then, crucially, you test the hypothesis (and eat the dinner) and see if it actually works for you.
You become a "paleotard" when you start pretending like you actually know how paleo people lived and begin trying to emulate it. We don't know to any degree of certainty and emulation on the basis of "they did it" is not well founded, especially given the massive variety in human behaviors across populations. I've noticed that this reasoning is often used as a way to justify any random bias a person might have (Melissa's answer about polygamy highlights a perfect example), which is just bad philosophy. Test those ideas, don't blindly follow them because you think they're paleo!
Most paleorific thing I've done: Dunno. Since the above is my philosophy around the whole thing, I guess I haven't done much on the basis of "following paleo". To date, my paleo interventions are:
1) Wearing moccasins. Appears to have solved a weak arch thing I've had since puberty. Win.
2) Stopped using soap except in extreme cases. Appears to have had zero effect on cleanliness/body odor/etc. Removal of unnecessary activity = win.
3) Zero carb/VLC/no starch/no fiber. Appears to have successfully controlled Crohn's. Ongoing experiment, though. Not ready to declare a win, although I'm excited with the results. It's also an extreme variation of paleo which you wouldn't settle on unless you limited your paleo research to Stefansson and the Alaskan Arctic Inuit; it took modern feedback to justify it in my mind.