How about the voting staying as it is, but a facebook-esque "Like" thing put on the comments. This way, people can "like" a comment without having to resort to voting. Their intepretation of voting stays the same.
People are familiar with the "like" concept, so such a move would be intuitive to a lot of users. It would also ease the difficulty in understanding the voting system.
I don't think of 'like' and 'agree' as synonyms -- in fact to me 'like' more closely means 'informative contribution that should be highlighted' than 'agree'. So I support the idea of 1-click expressions but think they need to be labeled/presented carefully to encourage desired meanings.
People are familiar with the "like" concept, so such a move would be intuitive to a lot of users. It would also ease the difficulty in understanding the voting system.