To me what makes a good open level is this. I go into a level and I look and go is there more then one way I can tackle an encounter. Every Halo game but 2 & 4 had a ton of those type setups. Even in the hallways of Halo 1 you regularly had multiple ways to handle an encounter. You'd come to a room. You could go left, right, through the middle, jump up on top of some of the walk ways grunts would use. You had a lot of options. Sure the game was funneling you from A to B but it made you feel like you weren't just on a pre determined path the whole time.
Now I'm not saying Halo 2 or Halo 4 did not have those moments but they were few and far between. I still liked both games campaigns though. But with both I regularly just had one route though an area. Another thing that I think hurt both games, Halo 4 more then 2 mind you, was that you regularly went here and pushed button. Then went where and pushed another button. Other games had some variety of missions in that you'd have to go shut down generators with your shields, rescue some members of the crew, race through a location to get to an item first. That's another reason they felt less repetitive. I do hope and I do think that 343 probably have realized this based off the footage and details revealed. Also I can say without a doubt the game is going to blow a lot of people away I feel. I was a huge skeptic and a 99 percent certain not buy. But I've had my eyes opened a bit and preordered the hell out of the sucker.