JPG is about inspiration and the joy of photography, so keep that in mind when commenting. Be respectful and offer encouragement; don't be a jerk.
Linking to or briefly quoting from other sources is fine, but your words should be precisely that — yours.
You're responsible for what you say
Your name is attached to all comments you write, so don't write anything you wouldn't want everyone to see.
Use personal messages for private conversations
Don't let a discussion get out of hand. If you need to have a private conversation with someone, write a personal site message to them instead.
Don't plaster the site with comments that have no relation to the subject, such as convincing people to vote for your photo.