I am not re-igniting the Great Food Wars of Spring 2014 - just to make sure, I've put loads of water on any ashes and coals that may remain.
That being said, it's pretty rude to say "Wow, that looks gross!", or "You eat weird stuff!" (salads are not weird - they are just vegetables), "You don't need to eat that,", or "Why would you eat that?". If you don't know what something is, think of a polite way to ask if it's appropriate. Friends, family, and coworkers you're on good terms with probably won't mind, strangers might think you're being strange. Or rude.
"That looks good/yummy/tasty/pretty." is acceptable. Just be nice. If you have nothing polite to say about someone's plate, keep it in your head. Or on twitter. Someone eating lunch doesn't need to hear about how you think it looks/smells like vomit.