What are the consequences of message ratings, protection and dissapearing messages.

Someone approving your message gives you a heal (as well as a warm fuzzy feeling).
When the message pops up, there's a glow about your character that reminds me of the glow you get when drinking, or the glow from life leech effects.
The approval might also refill an Estus Flask use, I'm not as sure about that.

I haven't gotten any disapproving votes yet, but nothing happened from disapprovals in previous games. I suspect that disapprovals on a message may stop it from showing up.

Sources: From experience. Getting whacked, then seeing the "Someone has rated your message" notice and watching my health go up.