Halo 4 builds on the previous plots and opens the game into a whole new world--a must have game!
One of my favorite parts about Halo 4 is the epic presentation. This game isn't simply an action game with a plot--it's the reverse. To sugar coat the experience, 343 takes the best from COD (mechanics, cinematic sequencing--not scripting!) and combines it with the best of Halo. Truly impressive.
My final thought is this: the game is very emotional (in a good way). The previous games in the series had some emotion, but not to the level one experiences with Halo 4. When you finish the campaign, you'll find yourself thinking about the experience for a long time excited for what is yet to come in the series.