Ghosts, perverse family drama, and a vow of revenge: Hamlet is all geared up to be a traditional bloody revenge play… and then it grinds abruptly to a halt. The play isn't about Hamlet's ultimately successful vengeance for his father's murder at all—that's taken care of in about two seconds during Act 5. Instead, most of the play is concerned with Hamlet's inner struggle to take action. Our point? The play is a lot more interested in calling into question the validity and usefulness of revenge than in satisfying the audience's bloodlust—although, sure, it does that too. Shakespeare had a theater to fill, after all.
Hamlet deals with three revenge plots, all of which involve a son seeking vengeance for the death of a father. In the end, though, the resolution of each revenge plot highlights the inadequacy of revenge.
Hamlet's delay is what separates the play from other revenge tragedies; it's also what marks the play as modern.