[GordonS]

If I post to Reddit, I'm giving Reddit permission to display that comment. I haven't read the T's and C's, but my expectation would be that I retain copyright over my own comments.

I could well be wrong (probably, even :), but my point is that giving permission to use my comments on one site shouldn't automatically give carte blanche to anyone and everyone to use them.

[MauranKilom]

Well, you almost certainly give reddit a free, unlimited, global, transferable, [...] license to reproduce your comments. That's just standard, and it's unclear how they could display your content otherwise. Whether there is any way for you to revoke the license (either in the T&C or in applicable law) is a different matter.

[bagels]

Any sane tos gives them irrevocable, royalty free use of content you post.