Oversimplified answer: Lindy Infante's mindset vs. Dick Vermeil/MadMikeMartz's.
Both are hailed as offensive innovators, but I always thought DV/MMM did more to create mismatches Faulk could exploit -- lining him up wide, trying to get him out to the edge more often, etc. IMO, Infante (Colts OC, then HC) pigeonholed Faulk into more of a traditional RB role... he may have caught 50-some passes in most of his seasons under Infante, but I'd be willing to bet most of them were screens/dumpoffs.
Marshall's been one of my favorite players ever since ESPN started picking up his San Diego State games; he reminded me of Thurman, maybe even a little (gasp) better overall. (Faster, certainly.)
The game-breaking talent was always there, but I'm not sure he'd be a first-ballot Hall of Famer -- and there's no doubt about that, is there? -- had he spent much more time in Infante's offense.
Just my opinion...