The book addresses three themes: the fear of death, consumerism, and the growing influence of postmodernism in American academia and culture.
The movie essentially drops the last one (aside from a few scenes which revolve around Hitler and Elvis) which is strange because the last one is the thing that DeLillo predicted very accurately. The rise of postmodernist horseshit and relativism. The notion that 'your truth' was somehow as valid as reality. The idea that reality itself could not be defined.
The movie only tentatively touches on those things by making obvious jokes regarding the banality of studying Hitler and Elvis, of academic posturing and navel-gazing in general. Shame really. That's the culture we now have but the film shrugged its shoulders at that and focused on the trivial.