A very mixed bag. A lot of stuff has been done right, but at the same time a lot of stuff has been done wrong.
For every feature that works, there is another that reeks of 343's inexperience and, indeed, immaturity as a game developer. Elements of game play that have been staples since Halo: Combat Evolved have been altered or removed in a clumsy, transparent attempt to push the load-out customisation. The load-out options themselves have not been thought through as well as they needed to be, which has thrown off the balance of some game modes. Some new weapons and armor abilities are blatantly overpowered but hiding behind awesome visuals. Custom game options have been ruthlessly gutted. The maps are also hit-and-miss, with some rock solid, even great entrants mixed in with others that are filled with what are, honestly, rookie mistakes. And Spartan Ops, though an interesting and occasionally fun addition to the series, is ultimately a poor man's replacement for Firefight.
One poor change to the Halo formula that warrants a particular mention is the Ordnance Drop system. Unlike in previous Halo games, where weapons spawned on set timers and knowledge of the game and map was thus rewarded, in Halo 4's Infinity options weapons are delivered to you after you earn enough points to activate an Ordnance Drop. It sounds fun at first, but could be used as the poster child for 343's cool ideas that were not thought through properly. In addition to robbing the map creators of the ability to control what weapons show up on each map, there is now no need to truly contest the map and fight for the spawn locations. This was a key element of the Halo formula and it is extremely noticeable by its absence.
Another poor design element I feel compelled to mention is the poor menu design. Halo Reach had an interface that was logical and easy to read, two terms that could never be used to describe Halo 4's interface. 343 have, for some reason, settled on a tablet-inspired menu design that is ugly and inefficient, with a lot of effort wasted on fancy animations that add next to nothing, and are clearly just a cheap attempt at the making the game look "cool".
Perhaps that is the heart of the problem 343 ran into. A lot of effort was spent on doing things that were meant to be "cool", and not enough effort was spent on making sure these things actually worked or were even good ideas in the first place. The trouble is that they are the kinds of things that you hardly notice when they go right, but stick out like sore thumbs when they don't.
Ultimately, if you can look past the title on the cover, Halo 4 is a rock solid shooter that looks absolutely gorgeous, and is overall a pretty good first effort from a new developer. But the fact that it has the word Halo on its cover means that Halo 4 was always going to be subjected to a higher standard than a new developer would realistically be able to achieve.