I'm not excited for Reach because of any one Reason.
It is the whole package.
I get to play a hopefully great campaign, a fun and balanced online multiplayer(based off the Beta), play a sorts of crazy custom games, fight to survive with my buddies in firefight(now with Matchmaking and customization), build and play on my own forged maps(as well as the communities), and I can record all this insanity in theater mode.
Not to mention the file sharing and community features on Bungie.net.
Its stuff like this that will keep the Reach community alive and kicking for many many years. Its the reason why people still play Halo 3 despite both its campaign and MM being inferior to the prior installments.