Anne is perceptive enough to know that there is something wrong with somone who gets along with everybody. He is "entirely too agreeable." (ch. 17) There are good manners - the ability to be polite to everyone - and then there's the tendency to be all things to all people. No one is universally admired without there being some hiding of one's true self, in order to be universally admired. Though Anne sees no evil in Mr Elliot's character, she cannot help but see the inherent deception in such a character and she cannot like it.