Well the Flood are probably returning so I'd wager M for Mature. However, I don't see why the rating is important.
Halo was always a rated M game because of blood and violence, plus the Flood. Despite having a light opening, Halo has always had that ONE moment where s*** gets real dark real quick and the gravity of the situation comes down like the Master Chief in the opening of Halo 3. It hits you like a ton of armor that 'oh man, this person is DEAD and probably suffered agonizing pain for HOURS before finally passing on'. In Halo: Combat Evolved it was Captain Keyes, in Halo 2 it was the Prophet of Mercy, in Halo 3 it was Miranda Keyes and Johnson (though their deaths were rather quick, Johnson was physically tortured for probably near an Earth day by Brutes), and in Halo 4 it was Cortana who was LITERALLY dying while Chief was slowly dying on the inside because of Cortana. The rating told you some serious business was going to go down and it was probably going to tear you a part emotionally.
Halo 5: Guardians is the first T rated Halo that is part of the main storyline (Spartan Assault is likewise rated T) and honestly didn't really make me feel too much except for really bad for John. I mean, he's got a difficult choice to make and it's gonna hurt no matter what.
So, it's important because of a noticeable trend; M rated Halos had more emotional impact so far.