Last night's Doctor Who wasn't _terrible_, but it's a good example of this form.
So, the writer understands what shape they want the plot to take. And it comes in two parts:
A Haunted House
Spooky noises in the walls
When someone knocks on wood, the house knocks
Poltergeist-like activity, with the doors and shutters closing by themselves
People sucked into the walls
A tower that nobody is allowed to enter
Spooky sounds/music ties this all together
A child grown old because they are protecting their mother
Questions about how far someone should go to protect their loved ones
How much should a person constrain their own life to look after someone else?
However, they then completely bobble this. Rather than go for a nice simple vague explanation (trapped alien life force that's somehow been bonded to both the house and its inhabitants) they go with insects.
Insects that don't seem intelligent, but can close doors and shutters, and remove the hinges (which are wood???). And those insects sometimes turn a person into wood? And sometimes turn a person into energy to feed to the person they earlier made into wood?
The insects are summoned by noise. But is it knocking or a high pitched noise? And why do the insects knock back? And are they in all of the walls so they can close numerous doors/shutters at once? And why do they care if the same piece of noise is on a loop? And why are they sometimes sucking them into the walls and sometimes coming out of the walls and turning them into energy. The old man says that they're preseved in the fabric of the house, but also they're turned into pure energy to feed his mother.
Why have the "She's his daughter. No, really she's his mother." switch? What does that gain us, drama or emotion wise? And how do we go from "They're summoned by knocking" to "They're summoned by high-pitch noise" to "He can communicate with them/tell them to do things" to "The mother can wave her hands and have direct control of them". If we were going to be at all invested in the emotions or fate of either of those characters then we needed to start caring about them a lot earlier in the plot, and we needed to have their themes come through a lot stronger.
Instead of thinking "I have a thing that works this way (for instance, sentient wood) - what interesting things come out of this, and what kind of plot does it lead to?" the writer has thought "I want all of these scenes. If I wave my hands fast enough then nobody will be able to tell that they don't fit together well at all."
And this is common amongst people who don't actually think that science fiction makes sense, or needs to be coherent. Who haven't read/seen/written much science fiction, or who didn't actually follow the explanations and therefore didn't think they needed to make sense, because they didn't make sense to _them_. And you can get away with a certain amount of that - coherency is more important than making total sense, and you can frequently get away with a certain amount of handwaviness if your emotional plot comes together perfectly. But when your plot is basically _made_ of handwaving, and your emotional plot is compressed into only the final three minutes, you're left with a mess. Which is a shame. (Particularly as I enjoyed the first three episodes of the season, which also had plenty of silly handwaviness, but managed to still be entertaining fun.)
Original post on Dreamwidth - there are comments there.