A member on the Bungie forums made a thread that pretty much hammered why Reach's campaign sucked. After watching this video he provided, I see his point.
I don't think Reach's campaign was bad but I think it could have been so much more. I was dissapointed in the same way with Halo 3's campaign.
Both Halo 3 and Reach should have had way more epic battles like the one's shown in that video. I think that I level where you have to make your way from cover to cover while avoiding fire from a Scarab would have been epic. A vehicular battle that took place on a wide open desert (5x bigger than Sandtrap) would have been epic. I've been dreaming of something like that since the end of Halo 2. The closest thing I got to that was The Ark on Halo 3. The only thing it was missing was the quantity and quality of friendly AI.
Before Reach came out, I remember watching the vidoc on how Reach can have twice as many enemies on screen at once. I just remembered thinking that maybe one day...there would be a Halo game that could have battles similar to the collage promo ad of Halo 3. Dozens on enemies and friendlies on screen at once, blowing each other up. It still nevered happened.
Reach could have a lot of enemies at once on the screen but it still felt like it was all just small skirmished. I feel that the friendly AI should have done just as well as you and to couteract the ownage, make it so that enemies respawned a couple times in order to make the battle feel long lasting and epic.
Instead, we have had dumb AI in the past two Halo games (CE and 2 don't count since the AI was good for the time) that barely hurt the enemy and it always feels like you are a one man army. I know that's how Halo is supposed to be...but I thought Reach was aiming for a sort of "helpless" feel. Instead, I felt about as powerful as the Masterchief (who was a little too powerful since his allies sucked so bad) and it just destroyed the feel of the game.
All in all, the campaign was fun but still hasn't felt like much of a true change since Halo 2. If more content from that PAX footage had made it into the final game, I'm sure I wouldn't be complaining at all.