The lost baseball
The lost baseball represents the broader intellectual search of DeLillo's age group to find the same kind of compelling, universal meaning that they used to have when they were young, even from innocuous sporting events. DeLillo's transcendentalist account of this Giants v. Dodger's ballgame helps to show the universal draw of the sport just a few years ago.
The nuclear bomb
Both academically and culturally, the nuclear Cold War between Russia and the US represents the USA's adoption of the post-WWII worldview of nihilism. Nihilism is the belief that the typically accepted parts of human life that provide meaning are actually meaningless, and the only meaning the human experience is the defiance of will that existentialism provides.
The nuclear bomb is the perfect image of this, because (especially in New York) the Russian nuclear capabilities force the coastal parts of the US to deal with the seemingly inevitable prospect of Russian warfare.
New York street culture
The New York street culture is offered as a paradigm for conflict in the book. The marital issues of Nick and Marian Shay are contrasted with the street account of the young Cotter Martin. The comparison is that no matter what the ideal outcome is for any specific party, the true peace comes when neither party needs anything of the other. (Contrasted by the intensity of the baseball and/or the nuclear warfare capabilities).
Marital infidelity
Marital infidelity is offered as an allegorical element to help the reader understand the complexities of (especially Lenny Bruce's arguments about) the Cold War. The distance of the relationship and the intimate nature of the drug abuse in the wife's account both allow the reader to appreciate the final, seemingly unaccounted for resolve of the marriage between the Shay's.
Drug usage
The drug usage in the book is a symbol for the meaninglessness of marriage, because it is indulged in outside of the marriage of the main characters, and also as an allegorical account for the Cold War, comparing the power of nuclear weapons to the erotic potential of heroin usage.
