[haasted]

But most of these additional features seem to be mostly about being allowed more advanced ways to contribute to the community, e.g. editing other users' comments, locking articles etc.

None of the features (that I know of, my SO karma is nothing to brag about) make Stackoverflow more USEFUL to the high-karma user, than to the low-karma one.

[jtheory]

Slashdot experimenting for a while with letting high-karma users disable ads.

They didn't disable them by default -- they just started showing a checkbox in the sidebar providing the option as a thank-you for "your contributions to the site" or something like that.

I don't see the checkbox anymore (I never bothered to check it; I don't visit much nowadays), so perhaps they decided the experiment wasn't a success.