I'm not sure if this belongs in the campaign forum or not but I'll post it here anyways.
WARNING: SPOILERS ARE IN THE NEXT READING, IT'S HIGHLY SUGGESTED YOU BEAT THE GAME BEFORE READING
It seems like the master chief didn't act like master chief at all in this campaign. He's usually the soldier that's willing to risk anything, which he did. However, it seems like he was really different in this game. I usually see chief as the prime source of bad assery and mystery. In this game, he almost appeared to be a little bit of a wimp, not completely of course he was still really bad ass but still, you can't say you didnt notice. I understand that they were trying to show his human side, but at the end (SPOILER) he has that sentimental part over cortana. You can't really match the master chief with a sentimental moment.
If not almost everything in the game were changed the personality change might have been eased in but it almost seemed like we got a new character and we obviously got new enemies. He dealt with things differently than he normally would and he seemed a little too attached with cortana in the end, he blew off Johnson's death in halo 3 who was a much more enjoyable soldier and character than cortana but she really impacted him. Of course that's my opinion but I still think it was a little out of place.
With the fact that more spartans were shown in the game made the build up of chief's character kind of die down. It used to be cool being humanity's last hope and the only spartan around but the fact that more were introduced really ruined that felling for me. Don't get me wrong, I overall really liked the campaign I just didnt feel like it was a appropriate ending to the series I've loved since I was 5. Yes, I did see the cinematic after the credits, but it still didnt tie it in correctly.