The Head, Tony Little, was quoted as saying they want boys with 'something extra to bring to the party' and that could be any number of things, so it's very hard to define and very hard to know if your DS has it.
Enthusiasm and joining in are definitely important factors, I agree. They want boys who will use the school, and all that it has to offer.
You don't have to be an elite sportsman, but if you are then you will have fantastic training and facilities. You don't have to be a brilliant musician, but if you are you might find yourself playing for the Queen. If you are an 'ordinary' musician you will still be encouraged to play in an ensemble and perform in your house concert.
You don't have to have been head boy, or the coolest boy in your prep school. My DS was not in the alpha group at his old school because this was really defined by sporting ability, but since being at Eton if anything his confidence has soared. It seems as though they are really good at finding what each boy's strength is and encouraging and supporting him in that, rather than making unfavorable comparisons with other boys.
I think the school would really really like to attract more boys who are not coming from the Home Counties, Eton-boy-factory prep schools, but those schools are good at what they do and, in addition, a lot of potential pupils would never think of applying to Eton, so it takes time.