I think a moderator button which simply flags a post and logs the member who clicked it (or worse, logs no name), without inviting a report, would create more problems than it solves because it assumes moderator omniscience. Well, we're not, and in all but the most obvious cases we would therefore need to PM the members clicking the button to ask why they did.
Dox47 makes a good point about the filtering aspect too. If it was possible to click a button to flag any sort of objection to a post then we would be buried under a mass of trivia, and those things definitely requiring intervention would become more difficult to see.
So a descriptive report needs to be made at some point, ideally proactively, and the moderator attention thread seems to work quite efficiently for that - a report is made and posted to it.
Also, this thread is easily bookmarked and renamed "WP Moderator Report" in the browser:
http://www.wrongplanet.net/postt237032.html
Now it's easily reached from anywhere on WP; just click the "post reply" button to start making a report.
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Giraffe: a ruminant with a view.