AAaA! Isobelle Carmody is so totally the bomb. Tamora Pierce, Brian Jacques, Philip Pulman as well. Ursula Le Guin.
'Good' fantasy is when important issues in the world today are commented on/explored, in an imaginative way. I like to think that's the whole point of fantasy, it's like our Utopia, and things which can't be solved in the ordinary can be solved in the imagination because in a fantasy world the author makes the rules. Terry Pratchett, for example, has a discworld carried by four elephants on a turtle's back. Very imaginative.
But then like any good story, it can't be so much about the issue that connecting with the story is forfeited.
I really like Isobelle Carmody because her stories are so rich with symbolism and imagery, and yet at the same time the language isn't so complicated that it's hard to read, eg. EarthSea and Lord of the Rings.
However, the language in Harry Potter is almost too simple, despite it being a good story.
So in conclusion, I guess 'good' fantasy has enthralling and imaginative plot and characters that draw you in, while subtly commenting on social/political issues that affect us in the real world.
And needless to say, some form of MAGIC.