I'm a long time halo fan. Things that this game doesn't tell you is that it's just one really long mission so if you need to lower your difficulty you have to restart THE ENTIRE GAME, and let me tell you legendary is legit a challenge.
Another thing is they never explain to you the weapons and how different weapons are better against different enemies, just a giant trial and error if you don't look it up. The open world is an awesome idea and it's highly recommended that you explore before you go fight a boss.
I'm overall kinda disappointed in this game. There is no multi-player campaign (slated for I think March sometime), the tutorial is laughable as it explains basicly nothing. And some of the driving mechanics needs tuning. For a game that was a year delayed that really dropped the ball.
But, don't take my word for it, go play it and figure it out for yourself and hopefully have fun.