Many have suggested that something about Cymbeline, which flits happily between Augustan Britain and Renaissance Italy, doesn't hang together, but the problem is that it hangs together too well. The plot is impossibly complicated, and resolved too neatly. Clearly, Shakespeare's mode here is parodic.
Yet it has an earnest center in Imogen, who is a model of goodness. In Italy, her banished husband Posthumus makes a bet with Iachimo that he cannot seduce her; and indeed she is unassailable, though Iachimo steals her bracelet as proof to fool her husband. She charms everyone that she meets, from the Queen's odious son Cloten to Iachimo to Guiderius and Arviragus to the Roman general Caius Lucius, who takes her in his service while she is on the run, disguised as the boy Fidele. Guiderius and Arviragus, who also think she is Fidele, fall over themselves in praise:
GUIDERIUS: Were you a woman, youth,
I should woo hard but be your groom in honesty,
Ay, bid for you as I do buy.
ARVIRAGUS: I'll make 't my comfort
He is a man. I'll love him as my brother.--
And such a welcome I'd give to him
After long absence, such is yours.
And this is within minutes of meeting her!* By contrast, evil in Cymbeline is a weak affair: Cloten is a foul idiot who follows Imogen to Wales in order to kill her husband and rape her over his body--but mostly he wanders around the countryside, lost. Iachimo, as Bloom likes to point out, literally means "little Iago," and the bracelet he steals is a pale imitation of Desdemona's handkerchief in Othello. Cymbeline himself, who banishes Posthumus (he is not noble enough for Imogen's hand) and then repents, is a poor man's Lear. Only the Queen, who wishes to kill Imogen and install Cloten as heir, shows any menace, but she is early revealed as impotent: The killing potion she has procured is, unbeknownst to her, a harmless sleeping draught.
Many learned perspectives on Cymbeline, including Samuel Johnson's, have been negative, but I might suggest meekly that we need not always expect the bleakness of a Lear or Othello, and there's something charming about this play about a good woman in an essentially good world. Yet it seems mostly ignored; if you have heard any passage from it, it is probably this one:
Fear no more the heat o' th' sun,
Nor the furious winter's rages;
Thou thy worldly task hast done,
Home art gone and ta'en thy wages.
Golden lads and girls all must,
As chimney-sweepers, come to dust.
This is the elegy that Guiderius, Arviragus, and their foster father Belarius sing for Fidele/Imogen, whom they think dead. What death anxiety is there has been hidden, by quiet self-consoling ("Fear no more...") and one very cheesy joke ("As chimney-sweepers, come to dust") and yet I think there are worse things one might read at my memorial service. These words are false in Cymbeline, where every good thing survives; for us mortals they may be more appropriate.
*Funnily, Arviragus' promise to "love him as my brother" is ironic, and Guiderius' claim that he would "woo hard" if she were a woman is too, but in a much, much ickier way.