Halo 3: Hardcore gaming & social content
Halo 2 is still played quite a lot online, despite being over 2 years old now. It was a pretty good multiplayer, and that gives it a lot more longetivity than single player only games.
One of the 'against' comments was that it looks just like Halo 2, with very slightly better textures. I'm not sure this is accurate, but it misses the point. They've made no attempt to change the basic multi player, it is the most successful ever on a console, after all. Instead they've added demo recording.
Demo recording has been around in PC gaming for years.