I'm going to copy and paste (most) of my comment from the r/Halo subreddit. I've played all of the main Halo games, from CE to 5, and I enjoyed each one individually for what it was. That being said, here's my current impression of the art style we saw in the trailer:
"I'm not gonna lie, I'm not super thrilled with the armor design. All of this is just my opinion and entirely subjective, but I feel that the style worked well for Halo 1-3 because there was an understanding of how to make things futuristic yet recognizable, but without too much worrying about the details since technology limits at the time only gave so much leeway for character models. But then the tech improved, and I feel that ever since Reach there's been a push towards more detail to give a "gritty yet high tech" appearance, which I feel was for the best.
The armor I saw in the trailer made me happy because of the nostalgic memories of previous Halo games, but also a little underwhelmed because, for lack of better words, it's a high resolution reskin. I'm not saying that's the worst thing ever, but I hope there's more work done to it before the game launches. Things like the flat design of the Mjolnir's outer shell lacking any sort of texture and the plain, glossy visor gives it a very underwhelming look. I wouldn't say plastic like, but definitely not a combat exoskeleton designed over 500 years in the future. The undersuit looks leathery to me and less like a lighter secondary armor that is a compromise between protection and mobility.
I understand the H4/H5 art design wasn't everyone's favorite, but to me the armor shown in the trailer feels like too much of a regression into old art design and less like a compromise between the two styles. At the end of the day, I play the games for the story and the gameplay, so this won't really affect how much I enjoy the next game. But at the same time, I feel like this could be a slippery slope; allowing the nostalgia of the "best days of Halo" put rose-colored glasses on us could throttle the game and hurt the creativity of the developers if they're limited by trying to fit the "classic" mold. They should be making a game that we want to play, and it should be a game that is true to Halo's roots, but it doesn't have to be a reincarnation of Halo 2 or Halo 3 to be a good game.
To wrap it up, I have an unpopular opinion, and I'm not a very vocal part of the fan base, but I hope that by the time the next game is launched, the armor is a design that we can all appreciate on top of some great gameplay."