People who would never think they were capable of comment on, say, nuclear reactor design with an hours study never the less think they're qualified to talk about the health effects of xyz based on about as much.
Now, if only contemporary medicine were as capable of understanding its subject as nuclear engineering...
In general, objects which are deliberately designed by humans are easier to understand than objects which arose from evolution. This is true even when one tries to apply genetic algorithms to build simple circuits or antenna- trying to understand why they work can frequently be very hard.
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.