What IS a good plot, anyhow?

Plot is definitely the thing I struggle most with in writing. Sometimes I think I don't even understand what a good plot is! So I boiled some really good stories by other people down into one or two sentences. I think -- though maybe this is wrong -- that a plot that can't be summarized in one or two sentences probably isn't a plot.

So here are the ones I came up with. I was trying to get their essence -- if I've missed it, please correct me! And if you have more examples I'd love to read them.

THE PLOTS

A plane crash leaves a boy alone in the wilderness, where he survives until rescued.

A plane crash leaves a group of boys on an island where some kill others until (just as the pack is hunting the hero down) they are rescued.

A teenager who doesn't fit in travels in time to rescue her father from evil, finding love and self-acceptance along the way.

When a mother abandones her children in a parking lot, the oldest daughter leads them all on a journey that ends in them finding a home with their grandmother.

A set-in-his-ways hobbit goes off to burgle treasure for some dwarves; he comes home rich and unrespectable.

A bratty little girl loses both parents to cholera and goes to live in a house with 100 rooms all shut up and a garden. She tends the garden, makes friends, and turns into a nice child.

An adored only daughter is left at a boarding school, where she's the pet pupil -- until her father dies penniless. The girl is made into a servant until she is rescued by a friend of her father's.

Four siblings are sent to the country during the air raids; they find their way to another world, where they fight a witch, save the land from her power, and become its kings and queens -- and then come home, to find that no time at all has elapsed in their own world.

An orphan finds out he's the son of two famous wizards and is a wizard himself. He is taken to a school, where he is famous, too; there he makes friends, learns magic, and battles enemies.

A kind-hearted girl doesn't do anything to stop the other girls from being mean to a poor girl who claims she has 100 dresses--and then, when the poor girl moves away, is very sorry.

Two children who have lost their mother are excited when a woman from Maine agrees to come live with them and hope she will stay; she does.

A girl is blown by a tornado to another world, where she goes on a perilous journey to ask a wizard how to get home, not knowing that she had the power all along.

Do these sound good to you? To me, even all by themselves, they sound more promising than anything I've come up with -- but maybe that's just because I know the books have all done well!

PATTERNS?
The character can be tested, but stay the same; or the character can be changed by the adventure-- -- either can work. It doesn't matter. The character can have a goal (find that treasure!), or things can just happen to her (be sent or put somewhere by adults,often). Either way, the character is clearly defined at the beginning and then, because of who she is, one thing leads to another after the starting point.

Usually, but not always, the things that happen or that the character does -- the external circumstances -- are dramatic, exciting. But I think they can be small, too, as long as the emotional stakes are high (will Sarah stay to be our mother?).

To me, most -- but not all -- of these sound rich and satisfying. They have a kind of elegant simplicity and symmetry as well as high stakes....and most though not all of them would make me want to pick up the book.

Now if I can just keep working on MINE until it makes me feel the same way I'll be off to a good start.