I know there's going to be a ton of user reviews out there, but what's really blown me away about the Halo:Reach campaign is how well the cutscenes are put together in comparison to the rest of the series.  NO SPOILERS here at all, as I'll be talking about the structure and quality of the cutscenes rather than what actually happens in the game.  I haven't even finished the campaign, so I do not plan on ruining the experience for anyone.

The opening cinematic to the game is one of the more impressive ways that I've seen a game setting up the mood and tone of the story.  Overall, it's very similar to the first trailer of the game.  This is a grittier Halo experience than we're used to, and we're aware that it's not going to end well if we know anything about the Halo universe.  Given that reality, we get a wonderful set up to the campaign and a real feeling that we're part of a team.
 
Later on in the campaign, I was amazed by how Bungie decided to really show us the planet of Reach in all it's glory.  Most Halo games have involved Master Chief zipping around the galaxy to a wide variety of locations.  This has often happened at a frenetic pace.  I was very impressed how the cutscenes in Reach take their time to show the player the different environments and eco-system of the planet; it helps us to empathize with the ending that we know is coming.
 
Finally, I was surprised and pleased with some of the techniques used for the grand vistas we're treated to.  Rather than trying to render everything in-engine with polygons like in the past Halo games, Bungie seems to have gone with a more concept art/digital matte painting technique in Reach.  The level of detail in the background images in the cutscenes is astounding, and you really want to be able to take some screenshots so you can enjoy the detail later on.  
 
This amount of detail really brings the player much more into the experience, and we get more invested in the story.  I can remember the scene from the Halo 3 campaign where the Arc is uncovered.  Because it was done in the game engine, it really seemed dull and uninteresting.  That same scene made with the techniques used in Reach would have made it a much more  immersive moment.
 
There are other games out there that have more impressive graphical power, but Reach is very smart in the way it uses its resources to bring the player deeper into the campaign.  I couldn't wait for the next cutscene to treat me to something new.