I thought 343 pulled off something bold and a great addition to the Halo franchise. Remember, if 343 didn't change the enemy or layout people would have said its boring for copying Bungie. You can tell 343 understand the Halo universe, uncovering areas that have only been addressed such as the prometheans etc. The Chief and Cortana's relationship build on what Bungie left in Halo 3, and shows a rare insight into the Chief's 'human' side which is a theme central to the joke Cortana made of which one is the machine. Graphically speaking it is stupendous for a 360 title, with incredible views that are not all copy and paste. 343 have upped the ante when it comes to the knights, instead of brutes or elites having predictable behaviour and weapons. Knights are not so predictable, and are a worthy adversary for the Chief.
Cons: Of course this is not perfect, the new grenade is a bit of a gimmick, and although the narrative is hugely insightful regarding the prometheans, to the casual player or first time Halo player this would definitely be confusing to follow. Conversely, it is Halo 4 so why start here and expect to follow the plot entirely?
Final con (basically a pro), the game had only 8 levels and should have been longer.
I love this franchise I first played Halo on my Xbox crystal my dad bought me when I was 6 and been playing since.