There are some extras in Halo: Anniversary that weren't in the original but were in later games like Halo 3.

Skulls that alter gameplay when activated - a lot of them are in hard to get to places, so it's not just a matter of knowing where to find them.

Terminals are short videos that fill in some of the Halo lore, similar to the text entries in Halo 3. They also hint at upcoming events [ie: Halo 4 et al]. There is also a secret 11th terminal unlocked via the Halo Waypoint. - iirc you need all 10 in game terminals to unlock it.

I don't remember any other shooter from back in those days that let you do all the things Halo did - and so seemlessly.

I'd love to see an update of Halo 2 that actually includes the things they had to cut from the game due to time constraints.

Looking forward to halo 4.