Overtime mode has always been considered part of the main game. It's on the same disk, and finishes the story. I suppose on the 360 version they could have automatically started it if you finished the cases and were at the helipad on time, but that's just a minor gripe, certainly not worth making a fuss over.
As for the difficulty, you can't really make it any easier when as early as the first area you can pick up a weapon that kills in one hit. Get the real mega buster and triple booked chainsaws and even the bosses are a joke. I suppose it could be made harder if katanas and such 1-hit kill weapons broke faster and were less common, but then you can just adjust your own difficulty level by not picking them up.
The save system could use an overhaul and I applaud them for the effort, but to use it as a selling point seems like grasping at straws to me.
