I don't know how long Reach was, but I have played through the campaign countless times. Halo has this sweet quality that few other games have. It's not all that open, l but the gameplay itself is just plain sandbox fun. Halo 2 was a disappointment because they focused everything on the story and narrowing the playgrounds... taking away a lot of the emergent gameplay.
Halo 3 was better in that regard, but only Reach feels like Halo. With new enemies and environments that are said to dwarf anything in previous Halo games, I have high hopes for this.