I agree with you that all those characters you named got great character development. But I feel that Singer did a better job developing *all* the characters, all while making Wolverine a focal point.
I knew who Rogue, Cyclops, Storm, and Jean all were. Maybe some of those people didn't get a *lot* of screentime, but I felt the screentime they did get was of high quality. And that's one reason why I have to rate X-Men: First Class the lowest of the group, currently, because when I think about actual character, I just feel like the other films had more of it. By the time character development started getting really sacrificed (X-Men: The Last Stand, X-Men Origins: Wolverine), we already had 2 or 3 movies to establish who these characters were. Buy The Last Stand, I knew who Wolverine, Storm, Iceman, Rogue, etc... all were. And I thought the main new characters like Beast got enough development.
I'll agree that characters like Colossus and Kitty Pryde didn't get much development, but I'd still argue that Kitty Pryde got more development than either Havok or Banshee did.
I mean, this movie is called First Class, and while the characters who were developed were developed superbly, I'd actually like to get to know the X-Men who make up the title of the movie.