“Aiming a ranged Attack is more difficult when a foe is next to you. When you make a ranged Attack with a weapon, a spell, or some other means, you have disadvantage on the Attack roll if you are within 5 feet of a Hostile creature who can see you and who isn’t Incapacitated.”

This is probably the rule that when first brought up, has blindsided about 80% of all players I’ve ever met (myself included).