These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Ear Worm: There are some people who have not seen the show in decades but who can recall the theme tune just by seeing the title card.
Ensemble Darkhorse: Peter Scolari's Michael Harris was written as a one-shot character for a season 2 episode, but the producers liked him so much they brought him in as a regular for season 3.
Same with Larry and the Darryls, who were one-shot characters for the second episode and brought back little by little, eventually replacing Kirk as the Loudons' next door business men.
Fridge Horror: In one episode long before the finale, it actually occurs to Dick that he might be stuck in a nightmare, and he tries pinching himself. It doesn't help.
It Was His Sled: Since the final episode is often ranked very high on various "television's greatest moments" lists, it's pretty much impossible to not spoil it.
Rewatch Bonus: If you've seen the final episode and know that everything is All Just a Dream, the increasingly insane and unrealistic behaviour of everybody except Dick in earlier episodes makes sense.