...is the lack of vehicles.
Think about it: there are at least four classes of Separatist Capital Ships, Venator-class Star Destroyers, V-Wing fighters, AT-APs, Turbo Tanks, Corporate Alliance Tank Droids, the Droid Flyers used during the battle of Kashyyk - and we still never got Homing Spider Droids, either.
These are all vehicles which have more than bit parts - but with the end of the Action Fleet line, I don't think we are ever going to see any of these. The RotS line has been planned out, and after figure #56 we are back to different cards and mixing up all six films as far as figure choices goes.
It just hit me today that what I considered the coolest part of these movies, the spaceships, are sorely underrepresented in the modern-day collections. The RotS toy line totally hasn't accounted for all the vehicles we need.
And I am still really saddened that we do not have a single example of a real Republic armored ground unit for dioramas. We have Hailfire Droids and AATs, but nothing to stand up to them.