A lot of it. He was lead designer and project director of NV, meaning he either wrote, or at least green lit, most of NV's plot.
I just hated it, because it was beyond boring.
- I never understood why exactly I should care about going after Benny, especially since he didn't steal anything of value(at least that my character knew at the time), and given that he would be back as at base of power, and he already almost killed me once. Something even Sawyer admitted was a weak premise.
- I felt the entire "every faction wants YOU" thing was not really well written. The Courier being asked to be the savior of every faction, simply because he walked into House's tower, is like making an actor the lead on a dangerous space mission to blow up a asteroid heading towards earth, simply because he did so in a movie.
-When it came to the actual war itself, nothing that would actually help the war, such as taking Nelson back from the Legion, destroying Cottonwood Cove, getting the pieces needed to repair the Legion's Howitzer, etc. etc. was ever brought up or did anything at all. All it was was a series of pointless "go talk to this faction" quests, most of which had VERY weak justification(Khans, Omertas, White Gloves) as for they the Legion is helping them, or was based on utterly illogical and shoehroned "WE JUST KNOW!" excuses, like the Legion's/NCR's motivations for sending you to kill the BoS.
Basically none of the plot actually made sense at all.