Instantly hot? Sexuality. Dressing in clothing that shows off their body, the way they smile, being flirty, a sparkle in their eye, etc.

Being hot is just about attraction to me. It tells me they can be fun. Now, you can tank that hotness in an instant by being rude to a waiter, being shallow, unable to hold a conversation, etc. In my experience hot people tend to have weaker personalities because they don't have to work on them as much. Hot is often not worth it, but it can be... hot.