Annalee Newitz has a fascinating, but I think ultimately incomplete, essay on the strengths and weaknesses of portal fantasies up at io9. I agree that heading into a perfect fantasy world doesn't make for terribly interesting fiction. But what makes portals interesting is the transition. You've got to choose to step across a portal and into another world, uncertain of what you might eventually find. Or you find your way across a threshold by accident, and no matter what you find at the end of your journey over it, the terror and anticipation of the journey through it are interesting in themselves.
As with most science fiction, the particulars of what you find at one end of a portal or what you leave behind at the other aren't particularly the point. What matters is what the traveler knows of him or herself, his satisfactions or dissatisfactions, his courage or lack thereof, when he takes that first step, and what he finds in, or lacking in himself, on the other side. It's easy for the vehicles for those revelations to be simplistic—unhappy modern lives or a kingdom falling under Christianity's sway—and for the means of revelation to be simplistic as well, be it a Christ-like lion or a matriarchy. Better inventions tend to lead to better revelations. But it's the desire for discovery that's often the most salient thing about the heroes and heroines of portal fantasy.