Analysis
Further thoughts on DFW's analytic talent over narrative was that in fiction he tended to over-expound into a sort of flashy look-at-me. They (Slate) quote him making that very observation about himself. Whereas with the non-fiction essay form the subject he was writing about provided certain natural boundaries. Reigned him in as it were.
And he was something of a tragic figure, no doubt. The account of one of his course outlines for his students was that it went way past what any 'normal' person would provide in terms of detail and aim. And while evidence of his special brilliance it was also a sign of excessive sensitivity/over compulsion. A mind spinning and spinning, relentless dissecting. Hence 'analytic' , referring to his fantastic capacity to perceive and dissect, with not such a great ability to create a narrative to contain that fantastic flow of dissected experiences.
On a related note, DFW when asked why he wore his bandana, said, 'to keep my head from exploding'... no joke there, from a fundamentally very bleak man.