Some people just don't.
In my experience, most people who don't care for Fantasy (poor benighted souls, like Miss Snark), generally lump the genre into one of two categories.
Either they say 'But it isn't real', by which they usually mean it does not conform to their worldview, or they say 'But it's just escapist', by which they mean it tries to avoid their worldview.
Liberals may attack Fantasy because they find it too 'traditional', while books like Harry Potter come under fire from Conservatives because it contains 'witchcraft'. Both groups fear that Fantasy may influence the reader's imagination adversely - a tacit admission of the power of Fantasy, by the way.
So how can we tell if Fantasy is 'good' or 'bad' (apart from the writing style)? Fantasy, after all, is produced by the writer's imagination, and imagination is a good thing, as I said here.
The 'Realist' style of writers like Stephen Crane, Frank Norris, or Emile Zola is firmly rooted in the naturalistic, materialistic worldview that arose around the turn of the last century: the world of Darwin and Marx (Karl, not Groucho), limited to descriptions and treatments of that which may be seen with physical eyes. But if we deal only with that which is real according to this definition, we perforce leave out much which is unseen, but which many people find gives Meaning to Life - like love, or God, or absolute truth, or a clear conscience. The above-mentioned writers produced works, incidentally (or consequently...), that were full of what most people would call the 'seamy' side of life, consciously amoral and devoid of 'sentimentality'. Their writing reflected their worldview. One can, of course, write 'realistically' without accepting this worldview. Read Dostoevsky.
But was it real?
Or was it a self-conscious attempt on their part to portray what they thought was reality? Those who make assumptions about what is 'real' will always be accused of Begging the Question, or at least get asked The Impertinent Question, i.e. 'Sez Who?'
Secondly, every work of fiction is by definition a work of imagination. The Writer projects not what is or has been, but what he or she thinks would, should, or could be, or woulda/shoulda/coulda been.
So my answer to those who say Fantasy is 'not real' is two-fold: Who Sez your 'realism' is really real? And, even your 'realism' is partly fantasy.
There are two ways also to answer the charge that Fantasy is 'escapist'.
The first is to say, with Bradbury, that Fantasy is not an escape from Reality, but a kind of back-door to it, Perseus looking into the golden shield to slay the Medusa. Those who write 'traditional' Fantasy know that one of the best ways to explore the invisible world, the moral and spiritual universe, is through the judicious use of symbolism. I talked about this briefly here.
Fantasy is not reality, it is a 'profitable invention', to quote Sydney, because it deals with Ideas, with Truth, and Good vs. Evil.
Or we can ask, with Tolkien, The Apathetic Question, i.e, 'So What?'
Who would, Tolkien asks, take issue with a prisoner who tries to escape, or failing to succeed at that, find something else to discuss or meditate on than bars and turnkeys? Yes, one may use Fantasy as an escape from responsibility, and this would count as bad Fantasy, but good Fantasy quickens the imagination, and creates longing for heroes, courage and beauty.
In a Zeitgeist that is naturalistic, materialistic, and amoral, Peter Jackson's vision of Tolkien's Lord of The Rings has re-kindled a generation's longing for something more than the dreary depressing doldrums offered by Charlie and Karl.
So how do I tell good Fantasy from bad?
By the effect it has on the reader.
Does it make vice attractive, scoff at heroes, encourage a vapid existentialism, and teach the reader to ignore the transcendent? Then it's Bad Fantasy, no matter how well written. Sorry, Robert.
Does it inculcate a revulsion against evil, root for protagonists that are virtuous (or become so), foster a delight in beauty, and fertilize the imagination with pregnant symbols of transcendent truth? Then it's Good Fantasy, no matter how badly written. Take heart, Terry.
To those who still don't like Fantasy, I wish you the best. I'll just pick up my pearls and go home.