OP, this won't work for a YouTube clip, but for general spoiler info, one method I often see is for people to write the spoiler info in WHITE text. That way, no one can read it unintentionally. Just enclose the spoiler text like this: [col0r=white] TEXT HERE [/col0r] but spelling "color" correctly. See example below:

Kevin Spacey is Keyser Soze!
Kevin Costner is actually a Russian Agent!
Bruce Willis is a ghost, and only the boy can see him!
Ed Norton is no choir boy; his alter ego killed the bishop!
Brad Pitt is just a figment of Ed Norton's imagination!
Darth Vadar is Luke's father!
Dil has a penis!

Highlight the space above to see the spoilers.

I'm not sure how such a feature would be programmed. I'm sure it would be possible to add, though a lot more difficult then I think it would be worth. But, even then I doubt we could expect much cooperation from our users to actually use it. So, it really seems like the effort/reward is not a good trade off. But, I'll keep it in mind for down the road.

I've been known to post the occasional Youtube video or two in a thread, but I know how much those can clog up a page's loading speed.

Can we add a spoiler tag that requires the viewer to click on a little button to reveal either the Youtube video (or the ending to something like a Mad Men episode, Downton Abbey episode, movie out in theaters, etc.)?

When you click on the button, it shows the stuff between the "spoiler" tags but otherwise the default is to hide the text between the tags.