"rationality" can be interpreted broadly enough that rational discussion of anything would count
"Rational discussion" is not rationality. You can very rationally discuss politics. You can very rationally discuss the life cycle of the cicada.
Truly "on topic" is content that helps the user to become more rational. Multiple definitions of rational apply: Being more practically effective counts. Being better able to sort through evidence counts. Meta-understanding on the meaning of rationality counts. Modelling what a rational agent might do in a given scenario counts. Figuring out what specific actions that one could take to achieve goals counts.
Anything, including politics, including cicadas, can be on topic as per the above criteria, or not, depending on context. Frowny on politics for its tendency to derail the original point. I think Lesswrong was intended as a rationality training ground.
But practically speaking, I think the votes decide, but from a standpoint of policing the boundaries this is what I'd encourage.
That seems quite a bit more restrictive than what currently gets posted, no? (I ask because I don't follow the site that closely.)
For example, what would be inappropriately off topic to post to LessWrong discussion about?
I couldn't find an answer in the FAQ. (Perhaps it'd be worth adding one.) The closest I could find was this:
However "rationality" can be interpreted broadly enough that rational discussion of anything would count, and my experience reading LW is compatible with this interpretation being applied by posters. Indeed my experience seems to suggest that practically everything is on topic; political discussion of certain sorts is frowned upon, but not due to being off topic. People often post about things far removed from the topics of interest. And some of these topics are very broad: it seems that a lot of material about self-improvement is acceptable, for instance.