There are different levels of things being spoiled. There's
A) Vague summaries (e.g. "After trying to warn Princess Celestia about the return of the wicked Nightmare Moon, Twilight Sparkle and Spike travel to Ponyville.") Usually don't spoil much that you don't learn in the first 25% of the show/movie/book.
and
The Ending (e.g. "Twilight and her new friends learn about the magic of friendship and use the Elements of Harmony to stop Nightmare Moon and return her to the peaceful Princess Luna.") This is pretty bad, but you can at least argue that the journey is more important than the destination.
and
C) The entire plot (e.g. "After trying to warn Princess Celestia about the return of the wicked Nightmare Moon, Twilight Sparkle and Spike travel to Ponyville where they meet Rarity, Rainbow Dash, Fluttershy, Pinkie Pie, and Applejack. Nightmare Moon returns to Ponyville to bring darkness over Equestria. Twilight and her new friends learn about the magic of friendship by traveling through the Everfree Forest and use the Elements of Harmony to stop Nightmare Moon and change her back to the peaceful Princess Luna."). Why would you even watch it after reading this?
A is usually okay. If I saw a episode summary about the length of A (which is about what is on the program guide I have for my T.V) I wouldn't feel spoiled at all after watching the full episode.
B can be bad, but there's still much to be seen
C is just like, is there a reason to watch it anymore?
The worst I have ever been spoiled of something was B, before I went back to finish reading the Harry Potter series. The books were still enjoyable, but I kept feeling impatient to reach the end because I already knew what it was. Since then I've been trying to avoid that sort of thing for everything else I plan to watch or read.