Here's an interesting panel from this year's
Gencon -- shared worlds.
This could be taken from the perspective of the reader or of the writers and editors. What makes for a good shared world that you'll enjoy reading more of, or how does one create a shared world that multiple writers can work in.
From the professionals' side, one of the first things that must be taken under consideration is the issue of copyright and intellectual property. It's less of an issue when one is using a 'verse that has already fallen into the public domain (King Arthur, Robin Hood, etc), but if the shared world will involve newer materials or a world created especially for a shared-world anthology, it's absolutely critical to nail down the limits of how the invited authors may use the existing IP, and the status of any new IP they may create in the process of writing their stories.
A number of authors have taken some segment of an existing fictional 'verse of their creation and opened it to other authors, either by invitation or in open calls for submissions. This really took off when Larry Niven got tired of fielding constant requests for more stories of the Man-Kzin Wars, an era that was frequently referred to in his existing stories, but almost never actually seen in any of them. He'd averred that he had no military experience and thus felt unqualified to write stories of actual warfare -- but the fans remained insistent.
In the mid-1980's Jim Baen offered a suggestion for a compromise -- would Niven be willing to open that very specific segment of his Known Space 'verse to stories by fellow professional authors, to produce an anthology of stories of the Man-Kzin Wars? Niven was amenable, and the result was a glut of stories of such excellent quality that the anthology immediately became a series of them. As things progressed, the invitation was extended to a wider range of authors, including semi-pro authors and some participants in various writing contests Baen Books held. There were a few hiccups, such as some writers who didn't seem to understand that Known Space aliens were not Bumpy Forehead aliens that exist to hold up a mirror to human society, but on the whole it proved successful.
In fact, it proved so successful that Baen Books began a whole program of teaming up new authors with established authors to write collaborations that would help the new author's discoverability and fan base. And it made it possible to turn Eric Flint's novel 1632 into a giant shared world with hundreds of stories by fans professionally published. Although many writers were happy just to play in his sandbox, a number of them have gone on to establish professional careers writing original-universe fiction.
However, that route's not without pitfalls. The long-running Darkover anthology series came to a sudden end in the mid-90's after a dispute about how much credit a fan writer should receive for the incorporation of her ideas of a certain event into a pro novel MZB was working on. Recently new anthologies have been coming out, but involved a strictly invitational format with only known professional authors who could be trusted to let things go to their heads. The Marion Zimmer Bradley Literary Trust has language on their website stating that it is illegal to even write a Darkover story for one's own private amusement -- although it's unlikely that criminal copyright infringement charges would be pursued for a story that remains in the proverbial "dresser drawer" (literal or digital).
That raises a related IP question -- what is the IP status of elements that the participants create in the process of writing their stories? Frex, if an author creates an original character for the shared-world story, does that character become part of the shared-world IP, or does it remain the property of its creator? This is important if a shared world is not a one-off anthology, but becomes a long-running series. If Author A created Character X in Book 1, can Author B use that character in a story in Book 5 without needing to obtain formal permission from Author A? And if Author A later writes an original-universe story or novel and has a character who has the same name and basic characterization as the character in Book 1, is it necessary to obtain permission even if there is no reference to the shared world?
Many of the earliest shared worlds grew out of informal writer communities, and as a result everything was a "gentlemen's agreement" or "handshake contract." HP Lovecraft encouraged writers of his acquaintance to drop references to his Great Old Ones into their stories to create a sense that his creations extended beyond the edges of the pages, and thus increase their power to chill. However, particularly after the mess with the Darkover anthologies, shared world creators have come more and more to need to spell everything out in a formal contract, and even to require potential contributors to sign a preliminary agreement detailing what existing IP they may use and the status of any IP they may create for their shared-world story before they can even pitch an idea.
Given how contentious things can become, it's probably wise to have these questions worked out and set into proper legal language even if one is creating a brand-new shared world for an anthology, even one of writers who are on friendly terms. Just because you're all on friendly terms doesn't mean your heirs will be -- and non-writers often have strange ideas of the relative value of literary IP, as opposed to media IP. It'd be a shame to have to take a successful anthology out of print because Joe Author's heirs think they can extort a six-figure payment to make them go away.