I couldn't answer the question, so I went and did a survey.
Here are some answers people gave...
La-las are weird people, with no color co-ordination, they speak with a weird accent, they have weird stuff dangling everywhere, they always hang around other la-las, and they closely resemble Taiwanese fashion.
La-las wear super baggy jeans, flip-flops, big shirts, their belts dangle, and their hair is really 'Chinese'.
La-las are people who are trying to be someone they're not- in other words, wannabes.
The La-la fashion was originally bell-bottom pants, but it's sort of.. mixed in with other fashions, so I don't know what it is now.
La-la girls are girls who aren't Japanese, but dress like Japanese. (I'm assuming this person was referring to Goth-loli)
La-las are gay.
Besides those answers, a large number of people also said that they didn't know how to explain it.
As for me, I think that a typical la-la person would have a hairstyle that you see on almost every Taiwanese star and has a bad attitude because he thinks it's cool.
Of course, I believe in one's right to choose how to act, so I don't mind 'la-la' at all. However, a large number of people also said that they dislike la-la people a lot.
It's a little hard to explain the whole la-la thing with text.. I guess it's one of those things where you have to be around to understand. ^^;
