Well, people get uppity about different things.

While, being an episodic show, the plot reveals tend to be things that don't change the universe in major ways, they still have an in-episode arc. Some people prefer not to risk finding out the resolution to an episode before getting a chance to watch it. For example, a recent episode was a riff on "Murder on the Orient Express". (Although, being a show about cartoon ponies, the crime was cake-eating, instead of murder. I can't believe how absurd that sounds, written out.) So even though it wasn't a twist on the order of something in Dr. Who, it still seems reasonable that some people might want not want to know who-done-it before they even get a chance to see the episode. Even for episodes that are NOT framed as mysteries, some people prefer not to see screen captures of plot points in advance.

As for simple vectors of characters being spoilered - the general policy is just "if it's within a few days of the episode it aired on, please spoiler-tag it, even if it doesn't seem like a spoiler to you." This "assume everything is a spoiler" policy actually makes a lot of sense to me, since it's the same policy I take with my friends when they talk about video games or movies I haven't seen yet - it's too easy for someone to accidentally spoil something, and it's too hard to guess what someone will or will not be able to figure things out from. So the safest route is just to say "screw it, assume it's all spoilers" and let people "opt in" if they don't care.

Mostly I think Reddit just needs an "official" way to mark posts as spoilers, since you're right, using the NSFW system is kind of a kludge. And sure, people could just not use thumbnails (and some do) but good luck convincing people to post their images inside of self-posts and lose out on all their imaginary internet points! :)