Bargains. -- It is often said that the play has three plot threads: 1) The pound of flesh. 2) The choosing of the caskets. 3) The keeping of the rings. Each one involves a bargain of some kind.
Money. -- Wealth, gold, borrowing, and lending are mentioned in practically every scene of the play.
Noting. -- "Nothing" and "Noting" were probably homonyms in Shakespeare's time, and the play contains a great deal of "noting" (overhearing things and drawing conclusions).
Wit. -- Both Benedick and Beatrice have reputations as wits, and those reputations affect how they are seen and how they interact with others.