So I've been thinking that there are different levels of spoilers. First you've got Episode Level Spoilers: Beginning. And then you've got Episode Level Spoilers: Ending. And then you've got a bunch of stuff I'd term Random Spoilers (includes quotes, character bits, clips, etc), and then you've got Arc Level Spoilers.
Episode Level Spoilers: Beginning means basically you know the plot of an episode before you see it. Or the premise, in any event. For 1969, you know SG-1 got sent back in time to 1969 and there they shall stay, unless they figure out a way to get back. And for "The First Ones," you know that Daniel is kidnapped by an Unas. Basically, you know the premise. What happens in the teaser and possibly the first segment, nothing beyond that. You don't know what happens in the relevatory scene, and you don't know how the problem gets solved. Now, in an ideal universe, the previews, the episode descriptions on stargates.tk, and things like that would give spoilers for the beginning of the episode and nothing more.
Then there's Episode Level Spoilers: Ending. This is where you know the key thing that's revealed. It might not mean the literal end of the episode, but rather... things like "the bomb doesn't actually go off because it requires proximity to the 'Gate, and Janet adopts Cassie." "The insanity is actually caused by a Goa'uld-killing device designed by Machello, and everyone gets better." This is the kind of spoiler that can really impair the watching of an episode. It's also the kind of thing for which the word "spoiler" was invented. Because, let's face it, you really don't want to know how the conflict gets resolved. The point of watching something for the first time is not knowing exactly what's going to happen, and I think people certainly have a right not to be told.
Then there are scene-level spoilers, the random spoilers, the really cute picture in the icon where it looks like they're about to kiss, "where there's a will, there's an or," the hug in Need... things that I'd seen/heard/watched long before I actually saw the episodes with which they were associated. This sort of spoiler also tends to hinder the enjoyment of an episode, because the pr3tty is the reason I watch stuff in the first place. And if I already know where all the pretty is, well, how am I supposed to get the fangirlish happy squee when I see it for the very first time? (Of course, I still get happy fangirlish squeeing when I watch the tag scene of Serpent's Lair, and I've seen that approximately twenty billion times, so...)
Then there's Mini-Arc Spoilers. Backstory. Weaponry. The second 'gate. Stuff that doesn't really spoil the enjoyment of the episode but when you include Daniel's parents' death in fic, you write "spoilers for Gamekeeper" because that's what they are. Sort of.
And then there are arc-level spoilers. Things like "Apophis isn't really dead after Serpent's Song, but he is after Enemies." Things like "Sha're dies." Things like "Daniel ascends in Meridian." Does this sort of stuff spoil the enjoyment of watching the series? Well... maybe. Except I had all the big spoilers of this sort before I watched, and hey, I enjoyed it pretty much. And I had the big spoilers for Farscape too, and I enjoyed it and managed to be surprised by the arcs, even though commonssense should have told me what would happen.
See, the thing about the spoilers that are out now is, they keep on getting associated with certain episodes. And that, to me, transcends the line between arc-level spoilers (yeah, the reason Apophis isn't there in S5? He's dead) and episode-level spoilers (SG-1 kills Apophis for good in Enemies.)
Anyhow, this makes me feel all icky, and I don't like getting spoiled, and I really, really, really don't like getting spoiled, so yeah, shutting up now.