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I think that's too personal a preference to make the default, and people would get confused. Imagine someone wanting to show a friend a funny video he found on reddit, and now it's gone.

They have already implemented something similar with the 'hide' button, which allows you to hide a story.

There's a very configurable script for fading seen/hovered/clicked links and it works on multiple sites including Reddit obviously and HN as well. This is more or less right up your alley:

https://github.com/Farow/userscripts/blob/master/reddit-fade...

There's an inverse feature if you have gold - it marks all the new posts since the last time you visited.

In prefs, you can hide submissions you've voted on. Turn that on and then start voting.

I assume because people will often go back and view comment sections on particular submissions multiple times.