One thing I read about the Blackwater episode - in the book King's Landing raises a chain around the bay to trap the fleet. The showrunners explained that the first thing HBO told them was there wasn't enough money to do the chain. Kind of a shame as that's how the bulk of the fleet was destroyed. So budget concerns drive a lot of decisions.
I was disappointed by the absence of the chain, too. It really did cement the brilliance of Tyrion's battle plan for the Battle of the Blackwater. And that particular tactic has a long history. I knew about it since I was a kid, and lived relatively close to West Point. I learned that when West Point was a fort for the Continental Army during the American revolution, there was a chain across the Hudson River below the gun emplacements of the fort. It was designed to keep the British from sailing up the Hudson River.
But I guess I can understand why the chain would have been too costly for GoT. The showrunners also had only one ship loaded with wild fire sail into Stannis's fleet. I think having a flotilla of barges sailing into was probably too expensive, too. Still, the show has absolutely been able to achieve an epic scope at times, and I think that it's still one of the most amazing series we've ever seen on television.