I have to go to bed soon, therefore I will not write up a long post but leave you with this short statement:
Yes, there is such a point in our rationality training. You underestimate the amount of work needed to get there. I do not think that I can reach that point within the next 30 years; and everyone on LW would have to reach that point to argue effectively. It only takes a few outraged posters to turn a thread into a shitstorm(see the comments and replies above).
It is indeed a word of caution, just like "do not play with electricity" is a word of caution. Grown adults should theoretically be able to handle electricity without getting electrocuted, but doing so(unless they're electricians) won't give them many benefits and there will always be that risk.
I believe that he suggested(he is not a moderator but a random poster making suggestions, remember) that jokes, humor and art not be posted here because this is not a website for jokes, humor and art, unless they somehow have to do with rationality. There are plenty of sites for such things if you really have a pressing need to discuss your love of the Mona Lisa or knock-knock jokes with people on the internet.
If you want my opinion, it's that a debate about Obama's healthcare reforms is less likely to improve rationality than a debate about the sequences or some other "traditional" topic. If you really want to apply your rationality skills in a real world context:
It's right there. Just switch off your computer, go outside and strike up a debate with someone in meatspace.
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.