Beware of Spoilers!

Over on Goodreads, I've been having a conversation with someone who thinks a couple of my reviews contain spoilers and should be marked as such. She flagged them, and Goodreads has dutifully added the spoiler alert, so no more discussion is needed for those two books. However, it brings up the question: what exactly is a spoiler?

Goodreads doesn't seem to have a definition.

Wikipedia: Spoiler is Genex slang for any element of any summary or description of any piece of fiction that reveals any plot element which will give away the outcome of a dramatic episode within the work of fiction, or the conclusion of the entire work. Because enjoyment of fiction sometimes depends upon the dramatic tension and suspense which arising within it, the external revelation of such plot elements can "spoil" the enjoyment that some consumers of the narrative would otherwise have experienced.

It's pretty clear how spoilers work in plot-driven pieces. Here are some examples:

• In Jurassic Park, the dinosaurs get loose and some people die, but the kids and the scientists survive.
• In Code Orange, it turns out he doesn't have smallpox.
• In every story about WWII, the allies win.
  

(I hope you don't mind that I've ruined all of these for you. Because I have.)

But what about in character-driven enterprises? What if the plot isn't what matters? What if I told you that in every Jane Austen novel, the girl gets the guy? You probably already knew that, and if you like her genre, you'd probably still enjoy the book.

Looking back at my reviews, I do have a tendency to make a vague comment about the ending. For instance:

• "only at the end do we find out how awful slavery was."
• "By the end, Holling grows up just a little"
• "though he eventually gets run out of town, he makes them start to think differently about people and cruelty. "

Do these count as spoilers, since they mention the end? Is it surprising to find out that at the end of a book that is described as "coming-of-age", the protagonist "grows up a little"? Or that in a book set in a colony of freed slaves, you find out how awful slavery was?

The last example does clearly give away part of the ending, but the point of the book is how the other characters think differently. Is the book ruined from knowing this? Is this a spoiler?

I'm looking for multiple opinions here, because I don't want one stranger's remarks to dictate the way I review books. I write reviews for myself, so I can remember what I read and whether I want to recommend it to anyone. Should I leave Goodreads and keep my reviews private? Should I mark them as spoilers, just in case? Should I ignore annoying people?

Edited to add: Goodreads has a "private notes" section. I'm currently making my reviews into private notes instead. If you want to be able to read them, let me know and I'll post them here.