Five things.

1. They're confident. Wallflowers are not charismatic. Confidence is like perfume or cologne: it's an intoxicating and addictive scent which permeates the entire room, refreshing those who sense it. People who know their own worth, who have nothing to prove, who are calm and forthright and voluble and perhaps even a bit brazen, who are who they are and don't give a split fig about what the world thinks of them, are irresistibly charming. We all aspire to be confident and we admire others' open displays of confidence. (Not false bravado, which is easily spotted, nor overconfidence, which is laughable and off-putting, but true self-assuredness.) Want an example? Donald Trump. During his campaign, he said things that made people's jaws drop, but he said them with such undeniable conviction that he persuaded millions of Americans that he was sincere and that he truly wanted what was best for the country. His obvious confidence in the righteousness of his words and cause (and his delightful lack of concern for his horrified opponents' opinions, and his refusal to pander to them) caught fire with the electorate and convinced the American people to vote him in. Whether you agree with Trump or not, his sincerity and unshakable confidence won him the presidency.
2. …but also humble. This is where Trump and many others fail the charm test. Confidence is one thing. It's easy to make an ostentatious display of confidence and talk up your own accomplishments. It's far more difficult to downplay them. It's hard to be the smartest and most accomplished person in a room and refrain from bragging or lording it over the others. But if you do so, and the others somehow discover what you've been hiding, you'll achieve hero status. Nothing's more heroic to a human being than a person who does great things and will accept no credit for it, and doesn't even talk about it until pressed. The doctor who discovers a miracle cure and refuses to patent it. The decorated soldier (or policeman, or firefighter) who insists he was “just doing his job.” The rock star who goes to neighborhood potlucks and never tells his neighbors who he really is (yeah, I'm looking at you, Jack White). True humility is depressingly rare, especially in this self-obsessed social-media-crazed society. Truly humble people (not self-deprecating or neurotic or negative people…confident, but humble) charm us with their purity and innocence.
3. They smile. You wouldn't believe how much charm depends on what you do with your face. Think of the girls and guys you had a crush on as a teenager. When you thought of them or imagined them, were they frowning or pouting or grimacing? No. They were probably smiling or laughing. You probably started crushing on them because you glimpsed them smiling or laughing. If there's anything that says “Hey, I'm nice and friendly and approachable” to our ape brains, it's a good, warm, honest smile. Sorry, pouty emo goth kids. Cheer up a bit and the cute preps will notice you.
4. They have passion. Think of the ugliest, grumpiest, most pathetic loser you know. Nothing penetrates the fog of negativity and depression which surrounds him. He's a total downer. Insults and self-deprecation dribble constantly from his mouth. His face is set in a permanent glower. His bald spot reflects the sun like a mirror and his gut oozes over his belt buckle. He is utterly devoid of panache, charisma, chutzpah, or charm. But then, one day, you happen to mention this depressing meatsack's favorite topic in conversation. And the guy just lights up. His eyes, which used to gaze dully from beneath his sagging brow, seem to blaze with a fire you've never seen in them before. He stops slouching and stands up straighter; he's a lot taller than you thought. His voice, previously a noncommittal rasp or croak, is suddenly full and deep as he orates. His hands, usually swinging listlessly at his sides, flit through the air like joyful songbirds as he gesticulates. His passion animates, invigorates, and vivifies him. And you notice that you're transported by passion too, and wow, this guy is a lot handsomer than you thought, and maybe he'd like to get coffee later…? That's what passion does. It charms and infects and transports others. A man with passion suddenly seems more alive, more human, more relatable, and we admire people who are human and alive, and perhaps are even a bit envious of people who can love something so much. The best example of this is Miles, Paul Giamatti's character in the movie Sideways. He's a miserable sod. Divorced. Alcoholic. Steals from his mother. Hangs around with a sleazy womanizer. Is balding and overweight. Quiet and shy and awkward. Temperamental and morose. Lies through his teeth, even to his friends. But when Maya (Virginia Madsen's character) gets him to open up and talk about pinot noir…suddenly he is transformed. He's poised. He's emphatic. He's confident. He's straightforward. He's passionate. It's like he's a whole new man. And he captivates his audience. Passion charms people. Nobody thinks apathy or disinterest is charming. People who have a passion and can confidently and humbly explain it to others will illuminate a room.
5. They ask a lot of questions. Nobody likes the boor at the party who does nothing but talk about himself, laud his own accomplishments, and bring every conversation he's involved in back to his favorite subject: himself. People like and remember (and are charmed by) the cool, confident, suave, yet humble gentleman who asked them a ton of questions about themselves and made them feel like they were the most interesting thing in the cool guy's life at the moment. Let me give you an example. There's a guy at my workplace, Roger. He works in the warehouse. He's never too busy to say hello to you. He's always genuinely pleased to see you. And if you stop and pass the time of day with him, he sincerely acts like everything you're saying is the most fascinating thing he's ever heard, even if he knows nothing about it, even if it's something mundane, like your recent trip to Scranton, Pennsylvania. Roger listens raptly, says things like “Wow!” and “Oh really?” and then, when it's his turn to speak, he never turns the conversation back to himself: he either makes some pithy observation about what you said or asks a question about it. You come away feeling like you've been really listened to…and soberly realizing how few people in your life actually do listen to you, or are genuinely interested in what you say. I would be very surprised to learn that Roger has any enemies, because he treats everyone fairly, never ignores or dismisses or tunes out anyone, always gives you his full attention, and never makes the conversation about him. His attitude (charm, if you will) is infectious.