I think Probending in Korra was stifled by its rules. It took all the creativity out of bending battles and turned it into an almost humdrum game of dodgeball.
I would have loved to see more supply line or ammo shortage stuff in GuP, but I think the skirmishes simply weren't large enough for that kind of thing. If there were multi-day large scale battles, that would be excellent. That said, I don't really understand what you mean by plot armour - the school generally got about as much damage as it dished out, with multiple tanks immobilised per match and some of them only winning because of the Flag Tank condition.
Lack of character development was, I suppose, inevitable in a show with a cast of around 25-30 or so, which is a pity. But it's not like you're really there to see the characters change, more for the character interaction. The type of show it is relies more on caricature than it does on realism, which is why the other schools (and indeed most of the characters of their own school - "Student Council Team", "Engineering Team" etc) are as they are.