Halo Reach was a true signifier to me. It was a signifier that Bungie was mentally and creatively checked out of the Halo universe, the game was simply souless. It was a good game at the end of the day but paled in comparison to even ODST, a game Bungie released a year prior. The Halo sandbox was starting to show it's age adversely.
Halo 4's campaign is mostly great. The story is generally far better than anything Bungie put together. However, there are two prominent issues with it however. One, the exposition is far too reliant on Halo's extended universe and doesn't front-load it in a way that would explain some narrative head-scratchers. Two, the three button "press this lever" objective structure is quite limiting and makes Halo 4 more linear than one would hope.
That said, this linearity also leads to just the right size for the campaigns enemy encounters, which are unrivaled by any other Halo. The moment to moment combat in Halo 4 really highlight the the uniqueness of the series "Combat puzzle" nature. The AI is ruthless and require you to be alert and skillful in a way that hasn't been seen since Combat Evolved. This campaign also happens to be the most consistent campaign yet, and although the three button structure is archaic in some ways, requiem is a world that somewhat alleviates that shortcoming. All in all, it's a very good setup for a trilogy with narrative hooks that could potentially pay-off big in Halo 5. Watching Forward Unto Dawn, before playing the game and the terminals after really do a lot of narrative meat.
Halo 4 is one of the best looking games (and audio/visual experiences)this gen. Absolutely gorgeous technics, incredible art design, evocative musical scores, vicious weapon sound effects, and superb V.O make Bungie's work with Halo 3 look like it was a generation ago. This leads me to the biggest change for the series, the multiplayer. Where Reach's multiplayer felt second-rate, 343's take on halo's crowning glory feels like Capcom's invigoration of Street Fighter with Street Fighter 4.
The maps, the weapon balance, specializations, packages, ordinance drops, and much more all contribute to a new Halo multiplayer that feels familiar in just the right way, yet evolved. Matches are far more frantic and dynamic, and battlefield moments feel organic and player driven. Halo's core multiplayer is a pure, nuanced omelet, and instead of 343 just adding chives to a cold dish, the decided to remake it from scratch and seamlessly add ingredients to naturally enrich the omelet. You simply won't find a multiplayer shooter this frantic, yet this intricately balanced on any other console this year.
Halo 4 is a traditionally executed Halo game with just enough changes to make it feel like its own beast. Traditional execution should not be devalued here. Halo is one of the most iconic franchises in gaming, living up to a legacy that literally defined a console and influenced a generation of shooters. It's a mammoth task for an unproven studio and would have crushed many a dev, but not 343. Halo 4 is the game you've been waiting for since Halo 3.