That one must prefix H,M,S,D,GM,SD, with D, was one of the ver first things I learned.
Since then, I've learned how amazingly supportive, generous, and life-saving MNers can be.
How commonly a thread can get descended upon with huge irony, swearing and name-calling, and not recognise the irony.
That some glorious swearing goes on and some threads are absolutely fucking hilarious!
That you can be called out on incorrect spelling!! Yes, I have seen it!
CF, a recent realisation
That leaving MN for any lengthy period of time means you lose your 2nd language skills very quickly and the dictionary has been considerably added to.
That many missed the memo about the tampons down the loo, or rather not.
That MN is so diverse.
That MNers never seem to heed warnings to not Google things things when no amount of mind bleach will ever be enough
That spiders rub their willies on your laundry hung on outside washing line
That NRTTATWT or NRTWT is heinous crime
That some expect babies to eat in toilets!
The reason we have pom bear emoji's, and the day that happened was historic. 
The existence of penis beakers and fanny gallops.
Well I could just go on....