The return of the Dark Archon unit would be cool as well.
I believe they wanted the units to be more unique.
In SC1 the Scouts, the Wraith and the Mutalisk could simply attack both air and ground.
In SC2 only Mutalisks can attack both as before. The Scout got replaced with the Phoenix, who can only attack air units, but can turn a ground unit into an air unit for a limited amount of time, enabling other phoenixes to attack it. The Viking can morph between ground and air and attack either ground or air. The main reason why the Banshee exists, is for the splash and the invisibility, I think. I'm not really using them when playing Terran, actually. Always prefered Vikings over Banshees.
Oh yeah, C&C4 was shit.