If you saw this post over at LiveJournal, you'd probably say, "Hey Kelly, what's with the Mr. Knightley icon? Isn't he an Austen guy?" Allow me to 'splain, and I promise that, while I open with a bit of a digression, it all ties in later.
One of the key themes in Emma, by Jane Austen, is an exploration of what it is to be a gentleman. The book compares Mr. Knightley (and yes, that name is somewhat allegorical), a model English gentleman, with Frank Churchill (who has adopted a different surname than what he was born with, and whose first name, which might imply candor, is instead associated with "French" manners - though charming, he is secretive and duplicitous and a bit irresponsible; he wants to live a life of idleness and pleasure (like the Prince Regent, whom Austen disliked), and he wants out of England ASAP). Mr. Knightley is honest and actually frank (he says what's on his mind), and he attends to all of his duties as a landowner, whether it's meeting with his tenants or policing and improving his estate or paying the proper calls on people.
A true gentleman, you see, attends to his responsibilities. If he owns an estate, he (like Mr. Knightley) must actively oversee his estate. He must be fair to his tenants. He must help to keep the peace (as Knightley does when the gypsies are in the area or the poultry thieves are about). He should attend to the poor (which Knightley does by indicating his familiarity with his tenants as well as with his generosity to the Bateses). In short, if he has any obligation or responsibility, he should keep on top of things and discharge his duties properly.
This notion of proper conduct comes from a long English tradition, and certainly extends to what was expected of lords and kings centuries earlier. And so we come to Prospero, who is rapidly established as being deficient in the execution of his duties as Duke. (Yes, I know he was the Duke of Milan and not the Duke of, say, Essex, but in Shakespeare's time, one could not create fictional dukes and kings in England and get away with it - one either wrote a "history", gussied up so as to stroke the current monarch's history and beliefs, or one set their play elsewhere.)
Prospero himself tells us that he neglected his duties in Act I, scene 2, when he unfolds his story to Miranda and tells her how he turned his back on his duties as Duke in order to (selfishly) pursue his study of magic - decidedly NOT a proper reason to abnegate one's responsibilities. In order to further his selfish goals, he hands the reins over to his brother, Antonio, who steers the state extremely well.
The abdication of responsibility is a big, bozo no-no, not only in Shakespeare's time, but for a few hundred more years in England and elsewhere. (As stated earlier, proper attention to one's duties remained a key indicator of mensch-hood in Austen's time, which is why characters like Darcy and Knightley and Wentworth and Edmund Ferrars and Henry Tilney and Edmund Bertram are good men - they live up to their responsibilities - whereas characters like Wickham and Frank Churchill and William Elliot and Tom Bertram & Henry Crawford are not, since they do not behave like "proper" gentlemen and they shirk at least some of their responsibilities.)
Add to this the somewhat repressive Christian society (and that was in flux in the time of Elizabeth I and King James, certainly), the practice of magic was also questionable, so the combination of abnegation of duty and the choice to pursue magic tend to mark Prospero as a questionable character.
His decision to cause the shipwreck that begins the play also shows questionable morality, and yet as the play unfolds, his character eventually reveals a strong moral center, apart, perhaps, from his treatment of Ariel and Caliban, although even then, he eventually does the right thing: he frees Ariel and leaves Caliban in peace (one hopes) on the island. He learns to forgive at least some of those who have wronged him - especially Alonso - and he opts in the end to cease his practice and study of magic and to return to his dukedom and to behave as a responsible gentleman ought. In this sense, one could classify The Tempest as a morality play, since Prospero learns to keep his emotions in check and to shoulder his responsibilities as he ought, but doing so would be a gross oversimplification, since this play is "about" a lot of different things - which is why Shakespeare remains relevant hundreds of years after his death, whereas Sir Dingly Dang ("You don't know, do you!") has been forgot.