Halo has stuck to it's own formula, which is exactly what you would expect from a franchise, but with other developers doing similar things has started to seem a bit samey.
One big thing was the Matchmaking system from Halo 2. Completely changed Console Multiplayer, strengthend (if not created, I'm not sure) Ranking and Skill based games, put in Gametype selection and the ability to party up as well as all the things you can do with that party. Nowadays if a console game doesn't have matchmaking it stands out and kind of makes multiplayer look like an after thought.
But I don't want the Halo franchise to constantly try to look fresh and exciting. What matters most is the core of the game and the bells and whistles come last and with a hollow core all the B&A won't help it at all. The thing is though, you only really notice the core changes. Halo has plenty of B&A changes like weapons, vehicles, powerups, Forge etc but the core remains the same. If Halo changed it's core just so it doesn't seem like it's not chnaging it would probably be worse, kind of like how ODST wasn't as good as Halo 3 even though it was more original compared to One and Two.
EDIT: What the hell is with so many people saying that Halo hasn't set the course for modern FPS. Shooters have been made much like Halo for years, nearly everyone cries that out. NO game is a direct Halo clone of course, everything brings something new (albeit small), but it seems like you are disagreeing with the OP just because he mentioned Halo.
Halo is a videogame!
No it isn't, it's a photo album with pictures of my dog and everyone knows that, you dolt!
What the hell...