Joss Whedon's film is pretty much how I'd like more Shakespeare movie adaptations to be - keeping close to the text yet not treating Shakespeare's work as a monument but as a living, breathing thing that contains more than enough room for an interpreter's personality and ideas without a need to insert one's raging ego for Shakespeare's. So, it's pretty much the opposite approach to Kenneth Branagh (whose approach to Shakespeare I loathe with enthusiasm, while I very much dig his Agatha Christie). Rather, this Much Ado feels to me like a writer really good at working with and subverting conventions paying his respects to another working writer really good at working with and subverting conventions.
And because Whedon is the great kind of working writer, he also knows how and when to step back and let the other people involved in a production give space to do the things they do better than he does, so another reason for Much Ado About Nothing's success is that it feels like a group effort in whose development the actors involved took as much part as the director, with many a moment that feels spontaneous, and many an actor I wouldn't have thought to be any fit at all for Shakespeare showing how to make the bard’s verse sound and more importantly feel natural in a contemporary (if not exactly naturalistic setting, for you'd need to rewrite the play to have it fit comfortably into something socially realist instead of emotionally realist) setting. Particularly Amy Acker and Alex Denisof - whose romance as Beatrice and Benedick the film emphasises for good reason in a contemporary version - are pretty admirable.
As is director-hat Whedon's ability to keep a film taking place in very constrained locations visually interesting and meaningful throughout in an elegant and often off-handed way that's never showy for showiness's sake.