Reach is everything Halo was meant to be.
Halo: Reach far and away surpasses any Halo title that came before it: better graphics, better acting, a more epic campaign which players can really make their own, armor abilities that add a new slant to the gameplay, better weapons (new and old) and a smarter AI. They essentially took everything in Halo 3 and ODST, ripped it all down, and started from the ground up instead of merely expanding on what was there previously. This led to a leap forward similar in magnitude to the difference between Halo: Combat Evolved and Halo 3.
I really want to focus on the two elements that go together and really made the game for me: campaign and the enemies. The campaign follows six SPARTAN commandos who fought during the fall of Reach, possibly the single largest and most drawn-out battle in the mythology (somebody correct me if I'm wrong). As such, there is a lot of room for some epic storytelling and stunning visuals, on which the writers definitely capitalized. We get a better feel for these super-men (and women) and realize that ultimately, they're not so different from us mere mortals. Thanks in part to their actors, we almost feel as the SPARTANs do, rejoicing with them for their victories, mourning with them for their losses. After the lapses last two campaigns, it's nice to see that Bungie's staff writers hit a home run with this one. My one complaint is that the campaign is short relative to other shooters (like Resistance 2 and Killzone 2), and while they put a lot of material into it, completion of the campaign leaves you wanting more.
Next there's the AI, and the enemies which it governs. Once again, the players have a reason to fear the Covenant, more specifically the Elites, who came back with a vengeance after missing out on the last game (we didn't see too much of them in H3 either). The 'Leets are considerably larger than they used to be, larger and stronger even than your biggest SPARTAN, and they reclaimed their fear factor from H:CE. They're also smarter than previously, commanding squads of their inferiors and organizing coordinated attacks against the players. On Legendary, the highest-ranking Elites are some of the toughest, most intimidating, enemies we've yet seen, and it is all the more satisfying when you put enough lead in one to take him down. They basically take a giant, wet, steaming $%^! on everything the Brutes could ever dream of being.
New weapons and abilities were just the icing on the cake for me, while not essential, they certainly helped. The classic weapons all got facials, but they're all familiar in the end, and while the new weapons take some getting used to, they easily become favorites within the first hour of use. New armor abilities and copious amounts of armor personalization make the game experience unique to each player, and it adds a new slant to the multiplayer half of the game which the community really enjoys.
All things considered, Halo: Reach is hands-down the best Halo title to date. It is an Xbox 360 must-have that will appeal to loyal Halo fans as well as newcomers to the Halo universe.