I think that the big sisters are going to add an increased element of strategy.
If you recall the Big Daddies were NEAR their little sisters but not always close enough to fully protect them. You could get to one and leave the big daddy wandering around looking around for his poor little sister the rest of the game. You didnt have to fight them all.. or any of them for that matter, and any combat that you did, was with the big daddy exclusively.
With the Big Sisters, if those are cages on it's back holding the little sisters, Then it means that the only way may be through direct confrontation, but instead of just breaking out all the stops and blowing the Big Sister away in an impressive display of fire power with your plasmids, you could potentially KILL the little sister in the process.. so tact may be required now, outsmarting and even disabling them to get to the little sister without killing her.
At least, thats my best guess...