1. "You look great, have you lost weight?" – The message you have just conveyed is that weight loss = good, and that the person looks better if they lost weight.
2. "You look like you've gained some weight." – But you wouldn't say this, would you? That's because our society views weight gain as always undesirable and thus a taboo topic.
3. "I wish I had [insert body part] like yours." – You just confirmed for the other person that you are looking at and scrutinizing their body and comparing it to yours. No wonder we are all so obsessed with our weight, shape and appearance.
4. "You look so healthy!" (to a person recovering from an ED) – I guarantee you that what he/she just heard was "You look fat." I know that's not what you meant, but that's how they hear it. Don't comment about their appearance. It's never helpful.
5. "You are too skinny, why don't you go eat a cheeseburger?" – Thin people don't like to be shamed for their weight either. Some people are naturally very thin, and others are thin for reasons otherwise explained. Either way, it's unkind to comment on their weight or to tell them to gain weight.
6. ANY comment about their body, PERIOD. It's none of your business. And commenting on a person's body (positively or negatively) reinforces the notion that we are judged based on our appearance. How about commenting on something else instead?