My DS is popular in that lots of kids like him - although he's not 'cool'.. He's started a few clubs at school, has friends in all year groups, gets invited to all sorts of parties.. I often have children that I don't know call out 'Hi, Mrs DS's Mum' to me!
I think, like a PP, he's pretty laid back, he likes talking to people and he's not shy about going up to other kids and asking to join in, he'll also see kids on the margins of groups and ask if they want to play too - I think that earns him some loyalty and a reputation as a 'nice boy'.. He's happy to hang out with younger and older kids and he really doesn't tolerate nastiness..
He's been buddied up with children with behavioural problems and has been 'successful' in bringing them into the class fold..
I'm proud of him because his popularity is a result of him being a kind, empathetic child - not because he's particularly cool or sporty or academic ..
His youngest brother is the opposite - he absolutely will not ask anyone to play and waits for them to approach him.. which they often don't! He spends lunchtimes writing stories, drawing or reading..