I stopped watching Castle for two reasons: it became too reliant on unbelievable dumb shit (Time travel! Telekinesis! Sasquatch! Ghosts! Psychics!), and because it drove story lines deep and hard into the ground (What's that? You were done with Beckett's mother's murder back in season two? How about we keep that story thread going for another seven seasons, now there's a plan!).
There's also the whole the writing went down the toilet (in case the time travel didn't tip you off). Take for example, back in season four or five, these two episodes; one where Beckett steps on a pressure plate that's connected to a bomb, and Castle decides to stay with her during the whole ordeal. Nothing really wrong there, but the episode that came after that (technically anyway, since due to the Boston marathon bombing they switched the bomb episode with the one after its air date) was largely about how she didn't know if Castle was really committed to their relationship, and almost starts a thing with Mr. Fantastic.
What truly killed the show for me was the main plot they laid out for whatever the most recent season was that Castle for whatever reason orchestrated his own abduction, but he can't remember that specific thing and anything remotely relating to it because MEMORY WIPE!!
Once Upon a Time suffered similar problems. Everything always tied back to Rumpelstiltskin, the evil queen is revealed to have done more and more pure evil things (such as massacre an entire village, and keep an unwilling guy against his will to use him as her personal sex toy [oddly enough, the actor in question plays Christian Grey now]), but the bad guys are really good y'all, let's give them another second chance!
Oh Arceus, the stupidity of these people, it's a wonder they haven't all choked to death on their own tongues.
True Love's Kiss (c) is the cure all to everything, and fine, why not, but it becomes unacceptable when True Love (c) is more common than sand.
And the extremely black and white morality. That made me yell so much. So, at some point there's this evil conniving ***** that's just merc-ing everyone, and no one can do anything about it because tongues, right? But at some point though, someone actually gets in in their heads to use this magical candle that'll kill her. Hooray, the day is saved as are whoever's left! Our hero is heroic!
Barrg! Wrong! She killed, so that makes her action Wrong and Evil, so now her Pure Heart is now tainted with Darkness! Even though she's shown to have killed a bunch of dudes in the past, this is So Bad, Y'all!
Oh, and then there was the whole "Frozen made stupid gobs of money, let's jump on that hype train", I quit when that was revealed.
(good thing to, since it seems that Cruela DeVil is their latest big bad. A lady that wanted to make a fur coat is the current antagonist in a fantasy based series.)