Whenever I think of the genre of fantasy, the first thing that I think of is the Lord Of The Rings series and the Hobbit. This series that was created by J. R. R. Tolkien sets the bar for the entire genre. A great way to judge new fantasy would be to compare it to Tolkiens stories. If they can hold their own, then chances are they will last generations. Im not saying that each new story should replicate the LOTR series, but if they book can compete with it, then chances are its a good story.
Good fantasy fiction creates a world where the reader does not question the world and the rules that exist within it. In the LOTR, the reader does not question that there are elves, dwarves, or even hobbits; the reader understands that this works for middle earth. The reader may question why it takes so long for the fellowship to progress or why Tolkien decides to spend an entire chapter on Frodo and Sam climbing some stairs, but never about the world.
Perhaps it was stylistic of Tolkien to write very long narratives, but in doing so he creates this fantasy land of Middle Earth that makes sense in its own wierd way. The reader does not question the world or its inhabitants because Tolkien so carefully created this universe. Everything belongs and has its own purpose.
Perhaps I am just being biased about the genre, maybe nothing can compare to the triology for me, but I do stand firm and believe that Tolkien is the grandfather of the genre. He set the bar high for others to compete with. There are several great fantasy writers in the genre, they just have big shoes to fill.
If all authors thought out their worlds and myths that they are creating as delicately as Tolkien has done, hopefully they will be able to hook readers. The story should be able to hold its own and be able to answer any questions that the reader may have while reading. "Why are there elves, dwarves, and hobbits living in Middle Earth?", "Oh yeah, duh! Its because thats their home and they have to live there because they dont live in our world!"