I suppose if you get deep into the semantics of "agreeing" and "liking," there could be made an argument for a distinction between the two. In my own head, I've been thinking that "agree" would be more suited for seconding someone's advice or assessment, and "liking" was less about agreement and more about either the style in which something was said or finding a non-mechanical issue interesting. I think you could use your judgment either way; functionally there is no difference.
As far as abuse of the system, these feedback options are entirely in the eye of the beholder. They can add weight to a recommendation or comment, but it requires a concerted effort to game the system, and it's up to the reader to decide whether these comments are worth accepting.
ken, unfortunately this forum software does not have the ability to provide inline replies like you're missing.