I am partial to original content, or settings which are inspired by other works but stand-alone as their own concept. It is both more freeing in terms of creativity and lore, often only having a limited amount of information present, but also in that it is mysterious; not everything to be known of its material in fact, [i]is[/i]. Depending upon the Game Master and the participants, the material might even be internally consistent, although this is far more rare - there tends to be one person who invents canon that does not mesh cleanly with the established, at least initially - it does happen in some unique cases.

I do not hate fandoms or established settings per say, but I do find they are greatly overrepresented by and large. You can only read so many entries on the same universe and concept and most, by virtue of being bound to their story, have overlapping themes with the content already established; characters taking the place of others, locations and events being the same, certain qualities being vastly overpowered or underpowered, et cetera.

The act alone of world building and developing your own story, or taking part of it, is a much more complex and difficult thing to do, as has been said before. Not only time consuming, but requiring a great investiture of creativity to do so. The easier route is to indeed either play within an established setting or cut desired elements from it and implement them elsewhere. 

I am entirely in favor of Game Masters creating original content over all else.