Much Ado About Nothing is so dramatic because it mixes comedy with serious issues, such as how accusations of infidelity can threaten to destroy a woman's happiness, and develops strong characters, such as Beatrice and Benedick, to challenge conventional notions of gender and romance.
Shakespeare holds the audience's interest in part through an exploration of gender roles. The witty repartee of Beatrice, who is sharp and wounding rather than soft, as a conventional woman was meant to be, means she can easily hold her own with Benedick—and makes her an attention-catching character. She helps offset the dully conventional courtly romance of Hero and Claudio—and call into question whether a modern partnership based on equality might be more interesting than a stereotypical romance.
Shakespeare also keeps the action moving using plot devices: Hero and Claudio might not be the strongest characters in the play, but audience interest is engaged when Claudio is tricked into believing Hero is unfaithful to him and humiliates her at the altar. Audiences also enjoy a good villain, and Shakespeare provides that in Don John, who does all he can to spitefully ruin the romance between Hero and Claudio.
The play also explore the roles of gossip and interference in love, not only in the negative way of Don John's malice, but through how the friends of Beatrice and Benedick use fake gossip, planned so the the two will overhear it, in order to convince each that the other one is in love with them. The happy union that comes out of this leads to a satisfying ending, as does the final outcome of Claudio and Hero's romance.
Shakespeare has tossed disguises, malice, gossip, masquerade balls, near destruction of a woman's reputation, mistaken identity, and witty dialogue into this comedy to keep the action moving and the audience engaged and attentive.