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.
Badass Decay: In Ancient History, Ghost Tucker isn't as powerful as Tucker was when he had ghost powers. Danny believes that when he separated the ghost from Tucker and broke Desiree's spell, it De Powered Ghost Tucker.
Broken Base: Despite the series' popularity, many fans are divided on the concept of Sam having ghost powers.
Growing the Beard: Following A Family Thing, the series seems to be coming into its own. Even Word of God has said that it was the story where the series would start really forming.
Nightmare Fuel: What She Wants revealed that all of the teens in Sidney's ghost Casper High will be there forever. Sidney only gets out thanks to Desiree hearing him while on a wish-granting spree, allowing him to graduate. Now think of other students who are probably miserable like Sidney was and can't graduate...
Word of God likes to wonder if they are real ghost students or rather just extensions of Poindexter, much like the band Ember had in her first appearance.
Vlad's Mind-Control Device: Some little tiny ring shaped device can be stuck behind your ear, where no one will see it. It will completely override your thoughts and turn you into a puppet. Then a ghost like Vlad can turn intangible, sneak up behind you, and put it there.
Squick: In-Universe example. Tucker and Valerie were visibly creeped out seeing Danny and Sam making out while Sam was dressed as Danny. Of course, many viewers have this same reaction upon seeing the pic the author drew depicting the scene.
Technology Marches On: It's clear that Fenton Tech will become more advanced in the future now that Jack and Maddie are alive in it, as shown when Future Danny and Sam's Fenton Thermos can be compacted into a disk form.