Depends on the age, I think, and to some extent the gender.
From age 4 to about 7 looks count imo. Girls who have their hair nicely done every day and who smile a lot are usually more popular than someone who looks dowdy, unkempt or glum. Among girls, the prettiest boys are sometimes very popular. Possession of a high status object will also make you the centre of much courting but the occasional challenge -- DD2 and another girl in her class had a battle of wills over a particular boy until DD2 agreed to 'give him' to the other girl, having found another trendsetting object. Also popular in this age group among both girls and boys will be the child who has attractive, smiling parents. Boys like a boy who has a smiling demeanour, is outgoing, keen on sports, shakes off injuries; among girls, they like a girl who looks nice and who is fun and outgoing, and not standoffish or gratuitously mean to them on the basis that they are boys (ewww) as some girls are wont to do.
Apart from looks from age 4 to 7, the children who are popular are not 'babies'; they can share, they can stand up for themselves, they can include others, they don't cry frequently, they don't tell tales to the teacher, they can deal with the give and take of the playground, they are not sore losers.. True charisma as opposed to being the queen bee is what I'm talking about here I suppose, and that extends all through their school days. The queen bee is not really popular necessarily, but she is feared. The child with charisma is popular and a leader.
From age 7 to about 12, the child who is good academically in school will be respected, but if that child has negative personality points (a 'baby', inclined to cry, boastful, too serious in demeanour, lacks sense of humour, is set apart culturally; isn't allowed the same music or tv shows others are because parents won't allow it, seems too attached to books, timid effort and lack of interest in sports, can't cope with losing or being wrong, mean with sweets, parent especially mother seen to hover and have pfbism, father seems browbeaten and down at heel, timid, etc.,) those traits will cancel out the respect and turn academic prowess instead into a stick to beat the child with. Another element is smell and cleanliness of the child, reputation of the child's house for cleanness or otherwise, reputation of the child's possessions, reputation of the child's mother as snobby or social outcast, mean, inflexible, welcoming, etc.
The child who shines academically and is also generous of spirit, smiling, able to do the give and take, has parents who allow some freedom and are not swimming against the cultural current when dressing the child, allowing interest in music, tv, etc., parents who allow a fair bit of socialising also have popular children. There are two such rarities in DD3's class, brilliant children in every way, one boy and one girl. They have contributed greatly to a fantastic atmosphere in the class. I will be very surprised if they don't turn out to be superstars in whatever avenue they pursue.