Campaign
343i have done a really great job in taking the Halo lore in a new direction. The campaign is tight, fast-paced and exhilarating and character development - a concept previously alien to the franchise - is mostly effective, creating an immersive experience. Gameplay is sharp and responsive, as are the plethora of new and old weapons and armour abilities that keep the game fresh but recognisable. AI are suitably intelligent, without being cheap (or bullet sponges like they had a tendency to be in previous titles) and the new enemies are a nice change to the classic Halo formula. The absence of Brutes and Drones is not missed, though the lack of diversity in enemy tiers (Generals, Majors etc) is noticeable especially among the Promethean ranks.
The biggest criticism I have is the main protagonist, whose rather bland motivation of "death to humanity" is only partially and unsatisfactorily expanded upon in the Terminals, a number of secret videos hidden throughout the game. His dull, textbook villain demeanour detracts from an otherwise great story and I can't help but feel that his role in the narrative was entirely replaceable.
Overall though, the Campaign is A* quality
Spartan Ops
Spartan Ops is the only place where Halo 4 really disappoints. A segmented, storied, multiplayer campaign-like experience designed as a replacement for the somewhat unpopular Firefight mode from previous titles, Spartan Ops lets down a decent story with repetitive missions and terrain reused and repurposed again and again. Despite the compelling story, which follows on from the end of the Halo 4 Campaign, copy-paste mission objectives only compound the poor netcode which makes cross-country or even cross-town play an input-delayed lag-fest.
War Games
War Games - 343i's new name for competitive multiplayer - is once again is fast-paced and responsive, but veterans of the franchise will have difficulty getting accustomed to new mechanics. 343i gets so much right here - fast kill-times, weaker grenades, nice movement and jump height are all welcome re-additions - but the implementation of several new features reminiscent of a couple of modern, class-based shooters I might name will leave traditional and competitive players wanting. Weapon balance is similarly poor, the DMR being streets ahead of most other weapons and incredibly frustrating with its ability to take you down with seconds across the map, and in conjunction with the new "Ordnance" system in which random weapon spawns are the norm gameplay can wildly transition from stagnant to overly chaotic in moments.
The removal of several Custom Games options from Infection and the notable absence of staple gametypes such as 1-Flag CTF, Assault and Race, is an unwelcome change to say the least and any fans of Halo mini-games shouldn't hold their breath; Custom Games options, notwithstanding the efforts of a number of rogue modders, are a serious and unexpected disappointment. It must also be mentioned that splitscreen play, while supported, experiences huge framerate dips and is frequently unresponsive.
However, it must be said that War Games in general is still a fun experience. While it lacks the depth of previous titles, classic Halo gameplay is more or less intact and while fans of competitive gaming may be disappointed this is still a great shooter by industry standards.
Forge, Theatre
The same cannot be said of Forge and Theatre. The Forge mode has taken one step forwards and two steps back with the existence of more Forge maps and a new "snap" feature apparently compensating for the lack of precision forging and a dull set of pieces copy-pasted from Halo: Reach. And this is not to mention the fact that, with 343i squeezing every last drop out of the Xbox's tired hardware, Forge maps (and splitscreen) are likely to make your console give up the ghost as framerate tanks, rarely getting above 20 FPS on more complex maps or split-screen games.
Theatre is where 343i seems to have entirely dropped the ball, featuring neither Campaign nor Spartan Ops recording and a 1-player limit on Theatre viewing.
Too Long; Didn't Read
Read the damn interview, I didn't spend an hour on this so your lazy ass could skip to the bottom!