I know a lot of work went into creating a more serious, "emo" tone, and the graphics are great, but just in terms of sheer gameplay, H4's campaign falls sort of its predecessors, IMO. There isn't that sense of vast space you had in CE or 3, and the geometry of the environments you fight in are kinda... Meh. Where are the huge, sprawling, hour-long levels? H4 felt like I was just playing chapters in a short story between cut scenes. As if the actual combat was just an afterthought. Also, at one point, you're just teleporting to different missions. I literally felt like I was on the stage select in Diddy Kong Racing.
There were a COUPKE cool parts. But there isn't any comparison to the long missions in Halo 2, (or CE or 3 for that matter.)
Don't get me wrong, I'm happy with H4 after being disappointed by Reach. The mechanics, the feel, the weapons.. It's a crispy game. But they need more of the passion and vision that Bungie had in regards to the Campaign missions.
