a very healthy balance of professionalism and levity
I've quoted this as I think this is the sort of balance we should strive for, where possible

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It's really hard to keep threads totally on topic - but there are areas where it is appropriate, for example help or technical threads etc... many of the threads on here are more light-hearted and a little off-topic is acceptable, particularly in Winter.
Keeping the forums rigidly on-topic is an almost impossible task, as it requires very heavy handed moderation and frustrates more members than it pleases (although on medical or highly technical forums, it may be appropriate). Ideally, we'll automatically come to a reasonable equilibrium where most threads stay mostly on topic if they're outside of the "general chat" section i.e. if it wonders off for a few posts, members will later on steer it back on topic. Selectively deleting or moving posts on a regular basis does ruffle a lot of feathers, as I've seen from when we tried this on the garden pond forum many many years ago.
Of course, if a thread diverges and would be better off as two separate topics, then us mods can step in and split the thread in two. Likewise, I'd hope that threads aren't dragged entirely off topic for long, but rather a new thread is created in the general chat section.
I much prefer light-touch moderation, unless things get out of hand - it tends to be the best balance for a successful and friendly forum.