Loose rivets - at this point we don't know whether the vibrations were felt before, at, or after V1 and the only human beings that could confirm that are dead. So despite our natural inquisitiveness and desire to solve the problem, we're basically fumbling in the dark until we get some kind of preliminary bulletin or press release.
So I think if we're not angling to make fools out of ourselves we should keep our powder dry and our minds open. That means cutting the personal cr*p and the pet theories and only contributing if it helps, whether a professional, an interested layman or just someone browsing though.
Come on people, right now all we have to go on is conjecture. The MD-80 series is derived from an old design, but it's a type just like any other. Pilots who fly the type know its idiosyncrasies just as pilots who fly other types know their idiosyncrasies. As such, comparison to A320/737/whatever is pointless and unhelpful.
Until we know, we can't really compare this incident to anything.