So, I'm one of those guys who pre-ordered the game for Sgt. Johnson.
So now I've played the game and my first impressions are good. It's different from Halo 3 for sure. If you're a badass you can still rush in and blow away everybody, but ideally you should try more strategy. Sneaking, gorilla warfare, and the helmet that detects enemies ads a new kind of strategy to the game. You got a little bit of sneaking action in the earlier Halo games, especially the first, with situations commonly showing up where you'll find Covenant who are on patrol or sleeping, but this game makes patrol situations more frequent, and the covenant come in big squads. The graphics are even better than Halo 3 if you can believe that. It was only a matter of time before the Silencer paid a visit to the Halo universe and it's done so in the standard issue Shock Trooper SMG. Your pistol is also pretty quiet. But the silence is understandable because sneaking is sometimes needed.
There's also the addition of health machines. These things are like the Health Packs from Halo 1, but in the form of a mounted machine instead. Oh, and they talk.
Possible drawbacks:
-You can't dual-wield in this one. It's somewhat disappointing, but also understandable. US Army soldiers never pick up an AK-47 off a dead soldier and shoot it with their left hand while still shooting an M16 in their right. The lack of dual-wielding is not as fun, but it IS more professional. It's more realistic, which is neat. You couldn't Dual-wield in Halo 1 and that's still my favorite of the series, so it's not really a major issue.
-Campaign issues. A friend of mine who's further in the game than I am had a glitch where whenever he beats a certain mission it goes back to the first level. He beat those same missions 3 times and the story didn't progress past that point. He didn't get to the end, the game just had an issue. I don't know if this will happen to other people or not, but it's a problem that'll probably be fixed in an update. Unless it's an issue with his Xbox. I haven't gotten far enough yet to confirm his story.
-Gun changes. Fans might notice small differences in the guns, such as changed plasma colors.
I'll add more as I play through the game. These are all just early observations.