The biggest problems with Whedon's modern-dress version of Much Ado About Nothing are inherent to the play. The humiliation of Hero, and Claudio's eagerness to believe the accusations against her, are always hard to swallow, though Fran Kranz as Claudio certainly sells the hell out of the wedding scene. And even Nathan Fillion and Tom Lenk cannot make the bumbling malaprops of Dogberry and Verges funny. (And why has Fillion been styled and dressed to look 50 pounds overweight?)
Whedon does make one directorial/interpretive choice that doesn't quite work. The film opens with a short wordless flashback to a one-night stand between Beatrice and Benedick, which is meant to give context to their constant bickering, but it also has the effect of making it even harder to sell the idea that Hero would be utterly disgraced as a Ruined Woman for having had a one-night stand of her own.
Speaking of Hero, newcomer Jillian Morghese is out of her depth in this company of actors. And blasphemy though it may be, I have never been fond of Alexis Denisof; he looks like a starving rodent and his reedy voice is like nails on a chalkboard to me. His performance is good and he certainly delivers the language clearly, but I longed for an actor with a more resonant voice and powerful presence in the role.
The rest of the cast is superb -- Amy Acker, Reed Diamond, Clark Gregg, Fran Kranz, Sean Maher -- and I'd single out Acker's Beatrice for particular praise; her "O, that I were a man" is ferocious. Whedon has skillfully edited the play to a manageable length. One of the play's more racist lines has been altered; another has been left intact, but with a nice directorial touch that draws attention to how awful it is.
Whedon also wrote the music, which is mostly unobtrusively nondescript, though his setting of "Sign No More" is very pretty.