Some of that was the format. Rowling actually put more about Dean Thomas in the first book, intending him to have his back story significant, all about his wizard father who was killed by the Death Eaters without his learning of it, or his mother either. But it looked superfluous so she had to cut it out. Maybe the editor would have insisted for an adult book as well, but the length restrictions for a child's book -- before she earned her leeway -- probably didn't help. As a consequence, Dean never become as important as first intended.
Though when she gained her length, it didn't always help. The whole Bill-Fleur-the other Weasleys subplot in Half-Blood Prince suffered because Bill was not a sharply drawn enough character. We didn't have an objective correlative, as T.S. Eliot put it, to explain the emotion he inspired. That actually improved on re-reading; knowing he would be significant later, I noticed him more in the earlier books. Still, the subplot needed him to be better drawn here, I think.
And even the members of Dumbledore's Army don't get deep, which perhaps reflects how few other students introduced in the earlier plotlines. Perhaps a necessary constraint since the last book was all the trio, all the way, but it did underscore how little the three interacted with others. Some had their moments -- Susan Bones, the one paragraph she had in the spotlight, intrigued me -- but all the three, all the way.