This is my opinion.
TLDR. A story that makes you think about it after you read it.
Generally, I think there are 3 ways things can go when you read a novel.
1. You dont enjoy it
2. You enjoy it but you have to turn your brain off
3. You don’t enjoy it but tolerates it
3. You enjoy it with your brain on
The first case happens because the story is not novel and not written well.
The second case happens when it is novel but not written well.
The third case is when it is not novel but written well.
The fourth case is when both of the things exist.
What is “novel” and “well written”?
Both of these terms depends on the reader here’s why.
Novelty depends on the difficulty of the reader to imagine the scenario of the story, which in turns means that if a reader is well read then he could imagine the story easily. It is why those JP novels with “strongest level …” are dull, just a glance from the title, an experienced reader can easily guest all the tropes and plot points. To a lesser extent “Reincarnated as …” is more pulling compared to “strongest level …” as you would need to think more to get a grasp on the story.
One of the best way to improve novelty is to pre-plan your story(or arc). If you found it hard to find a conclusive end to a story, it probably won’t work, it means that your story would be included with filler or non ending escalating threats.
On to the “well written”, probably the easier to quantify compared to the novelty.
Well written is the story’s ability to suspend disbelief. There are many common pitfalls and things to do, you probably could find these in a guide, for example.
-To do examples
Pacing; before dramatic moments build it up with something slow, slice of life, pov change, heartfelt moments as to not clutter the story.
Research; if you write something that leans on real information then it better be believable.
Lean on archetype; hero’s journey, wise old men, good vs evil, etc.
Words and Phrases; complicated enough to have you think on important moments, simple enough to understand
Mystery; keeps people hooked
-Not to do
Cliche; Mary sues, over used plot devices (young masters, etc.)
Repetition; no we don’t need the fourth scene of you describing preparing food
Unnecessary details; don’t bring up a place that’s gonna be relevant only 100 chapters later
As to the turning on or off the brain, it is easier to ignore suspension of disbelief when you don’t think.
For turning on the brain it happens when you relate with the story.