I consider six prime qualities to exist that make for a good plot.
For the audience to be able to root for the hero, it has to matter whether they win. There has to be something significant on the line.
Note that after a certain point, bigger stops being better. A story about saving the galaxy from certain doom is not likely to be any more powerful from that fact than a story about saving a few people. The reason is that the fewer people are involved, the more characterization you can give to each one, thus giving the audience a stronger emotional bond with each individual person on the line.
If the audience can tell what's going to happen next, it's going to be a lot less enjoyable for them. Imagine watching a recording of a tournament match in your favorite game. Wouldn't it just kill the experience if you already knew who was going to win and how?
This is part of why I wish people would write more tragedies. If there was any doubt in our mind before the end of the story that the hero would win and survive, it would make it feel a lot better when they do.
A good story is one that could plausibly happen in the world it's set in. It doesn't depend on coincidence or characters being stupid, and contains no plotholes.
A good story introduces or at least foreshadows whatever elements are relevant to the outcome of a scene before they come into play. If you fail in this area, the audience complains that it feels like you're making the heroes win by changing the rules at the last second and that the heroes haven't earned their victory. Not only is this unsatisfying in the moment, it destroys the future of the story because the audience can't trust you anymore. They're going to expect you to do this again next time, and so they'll never take the danger as seriously as they did before.
A common scenario is that the hero needs to be rescued. This is fine as long as you give the audience a reasonable way to see it coming. For an example, see my article on the Last Second Rescue trope, a subtrope of Deus Ex Machina.
Another way this flaw crops up is when the protagonist keeps secrets from the audience so that the audience experiences an illusion of tension when there really isn't any from the protagonist's perspective. This is never a good idea and doesn't even solve the problem it purports to solve. As soon as a self-respecting audience learns the truth they'll feel betrayed because not only can they no longer trust you as the writer, they can't even trust the protagonist. How are we supposed to be invested in a story when we know we've been lied to about what's happening in it?
This might sound like I'm saying something I'm not. Sometimes the protagonist comes up with a plan and then executes it in the next scene, and the writer doesn't want to have the protagonist explain their plan when they come up with it because then the next scene will be incredibly boring as the reader watches events unfold exactly as they were told they were. That's fine. Having the protagonist say they have a plan and then execute it without explaining it is fine. Having the protagonist clearly give the impression that they don't have a plan, that they feel cornered by the villain, and then reveal that they saw it all coming all along and actually have the badguy cornered - as happened in Solo - is where it becomes betrayal.
The last form I want to talk about is making major changes to the rules of the world late in the story. This one might be a bit too blatant an example to be very meaningful, but if you write 50,000 words of gripping hard sci-fi and then reveal that there's magic in your world, you've betrayed your reader possibly even harder than above. The first few chapters of a story are dedicated to setting up the world and telling the reader what is and isn't possible. The longer you give the reader to get used to your world, the more you lose the right to change it.
You could say that these three points form a kind of trinity: the plot, POV character, and world must all play fair with the reader. The reason I consider all three as the responsibility of the plot is because the plot is how the characters and world are presented (the plot is, in turn, presented by the medium). Having characters act believably and the world be internally coherent is not the responsibility of the plot, only them being presented in a way that respects the audience and their trust.
- Hero Agency
The hero must be the hero of their own story. Their choices should steer the plot. If they don't, why is the story written from their perspective, instead of someone who actually matters? This doesn't mean the hero should always be successful; on the contrary, good stories generally involve a lot of failure. But the direction of the plot - especially the ending - should still be a result of the hero's actions.
- Villain Agency
Like the hero, the villain should be one of the main forces steering the plot. Sitting on a throne giving orders is not the best way to achieve this. The best villains are proactive, taking action, adapting their strategy as the hero learns and responding to threats.
Saren from Mass Effect 1 is a good example of this. He's introduced in the first mission of the game by name, given lines, and shown to be villainous. He takes action and moves around throughout the story and the player has a run-in with him on the third-last mission where he reveals his motivations. The final battle and the boss fight against him is not in his lair, but in one of the first locations of the game where major plot events happened. He's there for the same reason you are: to do something by hand, because he's an actual person and not an abstract force who only works through minions.