Having just had a pretty deep Halo script and gameplay discussion with Deepblue, we wanted to share our thoughts. Since he can't, I will.
First some facts. Joe Staten wrote the script for Halo 2 but only performed a rewrite of Halo 3's script near the end of the project. (Joe told me himself.) This is why the plot and scripts of Halo 2 and Halo 3 are so different.
Blue and I (reads oddly, no?

) agree that the story isn't there in Halo 3.
The script only serves one purpose: to move the player from level to level. It also has problems with taking itself too seriously and that detracts from the experience. Halo 2 had many moments where the script was amusing and the characters were very much in character. Example: Sergeant Johnson's lines right before Metropolis:
And of course...
Instead,
Halo 3 takes itself too seriously. The game takes control away from the player often to tell the story via cutscenes and even then
its always about being huddled to the next area to do the next thing. The one revelation that occurs, the Ark, is short-lived because right after its activated you run through the Floodgate stage (15 minutes tops) and go through the portal to immediately rush to the next area.
And the game itself is too restricting. Halo 3's campaign is noticeably a very controlled experience for the most part. Unlike in the previous games,
the levels are engineered to push the player down a designated path with very little wiggle room. For me, this really hurts the campaign mode's replayablity. With Halo 3 I feel as if I'm playing the levels almost exactly the same way each time aside from vehicle sections where I have a choice of vehicles to use. You just can't no roam a stage like in Halo or The Silent Cartographer and play it differently every time. Even Halo 2's Delta Halo was explorable by escaping the main level areas.
And here's the other controlled part. Halo 3 more so than the previous two, really restricts the player. As a player, I want to be able to reach a spot if I can see and if it appears to be reachable. While it makes tons of sense for multiplayer maps, it doesn't for campaign. You can't have experiences in this game like you can in Halo 1.
And being 7 years later, why has Bungie taken two steps forward and then three steps back in that regard?
Obviously, this the way things are. These are just our observations and opinions. Honestly, I think Bungie has done their best work when they just work work work and push it out the door. I know that's not always best (BXR, anyone?) but its an approach I favor.
Halo 3's campaign feels like it was focus tested immensely and its really lacking. It feels dumbed down to the lowest common denominator and complaints many vocal players had (like playing as the Arbiter) were addressed when they necessarily shouldn't have been. The artists did a great job and some of the missions are fantastic thanks to people like Dan Miller.
But they still are a little short of amazing, sadly.
We wanted to post these thoughts to put it out there and possibly get some Bungie folks' attention. This discussion may not be new but we started talking about it as we were speculating on how great Bungie's new games will be and as to what will Joseph write.
We're really interested in them really making their next games huge sandboxes like some classic Halo levels were.