I actually would love for LMSs to have like/dislike (or digg/bury) buttons. In many rubrics in my online courses we are told to not just respond "agree" to someone else's post. We ought to be writing something more substantive. Of course this becomes tedious when you want to let people know that you agree, but the person has said things so well that you don't need to add more info (I find the "disagree" or "I agree, but..." proposition to be more conducive to writing longer, more thought out, statements.
I've been peeved when classmates have just written "agreed!" or "hooray!" or "you're right!" as a response because it clutters my mailbox...then again, if there isn't a like button, there is no other way of letting people know that they've struck a cord.