In the latest podcast (starting about 41:15), Jeff, Joel and Marco discuss adding a way for anonymous users to provide feedback to answers after all - recognizing the desire of drive-by users to say "thanks" when they find an answer that helps them. In that vein, a concrete suggestion:
Give anonymous users both the possibility of +1ing a contribution, and also adding comments.
Anonymous votes get turned into a comment:
anonymous upvote- they will not have any other effect on the contribution's score whatsoever
Only anonymous upvotes are allowed, not downvotes (effectively making this a "Like" button)
Anonymous comments do not trigger a comment notification
Anonymous comments have a lower removal threshold when flagged (say, by 2 users instead of 3)
Anonymous comments are hidden by default and have to be explicitly opened:
when shown, anonymous comments would be visible in the comments list, slightly highlighted:
the upsides that I can see:
It would be a way to channel all the "Thanks!" and "I have this problem too" answers and allow everyone their say, but without annyoing regular users and cluttering the site.
Users can point out blatantly incorrect or outdated content without having to sign up, and without having to earn the rep necessary to comment - a problem that is frequently raised on Meta.
the potential downsides:
Spam - a problem common to any approach that allows arbitrary comments from anonymous sources.
Because anonymously submitted comments get hidden, they will be subject to less community scrutiny than normal comments.
I think the potential upsides are big enough to make this worth trying out.