Halo: Reach is set on a planet named... Reach. Unlike last time around, you resume your role as a cyborg warrior. However, you aren't playing as Master Chief, but as another Spartan, Noble Six. If you paid attention in the other games, you know what Reach is. It was overrun immediately before the first game.
A cracked helmet lying in the middle of a field book ends the campaign. Despite your well-written teammates being picked off throughout the game, there never was a dreadful feeling of hopelessness. You are almost always fighting with a team. Although the other Spartans in your team have unique, identifiable personalities, it's hard to get attached to them, due to the ending. No matter what is thrown at you, you still make measurable progress and keep it. You evacuate civilians, defend a base, and blow up a huge ship!
Your armor has a slot for one of a dozen abilities. Sprint is the default, which I never swapped out except for a specific mission where I had to jet pack over a gap. I didn't notice any new weapons (nor missing ones), but like ODST, you can't dual wield any of them. The same vehicles also appear. (Although plenty of equipment doesn't appear in Halo 1, that is cleverly explained away.) Although I had night vision, I didn't rely on it as much.
Because this game takes place before Halo 2, you're fighting elites, along with the rest of the Covenant gang. (There are no Flood here.) In fact, Reach has the most diverse cast of Covenant enemies in the game, because some jackals are
African American black. They look like crows, seem to be moderately stronger, and lack that annoying energy shield.
Environments are reminiscent of the main trilogy. You fight through some canyons, cliffs, military bases, a city (though not destroyed like in ODST), and for the first time: SPACE! You fight near glaciers, but not on them, nor do you fight in rain. Additionally, I don't recall ever doubling back or fighting through the same area twice. Bungie have finally built a game that doesn't repeat itself. Too bad this is the last Halo game they've ever made.
Overall, this is a great game, even ignoring the fact that you're not Master Chief and you're doomed. With this, I am finally caught up on what the Halo Collection has released. The next installment, Halo 4, is set to be released on Tuesday. Despite not having the best reputation in the series, I'm looking forward to it.