- Space with limits. An island may be from less than one to several dozen square miles. That's a lot of room, probably with resources to find and use, perhaps allies to befriend and strange artefacts to discover and attempt to decode for clues. But at the same time, if a demonic monkey is hunting you, the island is still an island and you're going to have to find a solution to the monkey at some point.
- In-game rationales. Islands are a prefect excuse for stuff laying around for centuries unnoticed, or for weird monsters (that can't swim) to be roaming without the local bigwigs sorting it out, or for freakish and bizarre governments or cults that have been unmolested by concerned neighbours.
- Lack of reinforcements. The PCs can't simply "phone the police"; if there's a situation then they have to deal with it. The space on the island may give them more breathing space than in a dungeon but the setting gives a certain amount of focus and impetus.
When used as the setting for the very first scenario for a group of new characters the shipwrecked trope is one of my favourites as, in addition to the above, a shipwreck also:
- Explains why a group of potentially very different characters are together; even allowing for alignment combinations that would not work in other situations.
- Gives the characters an automatic sub-plot to deal with in that they presumably want to find a way off the island and back home but have lost the one they had (ie, the ship).
Islands formed the core of Sinbad's adventures, Conan had a few of his most memorable adventures on islands, and they naturally figured very prominently in Greek myths. Islands make excellent places to put the weird (of which I'll write a bit more next time) and certainly make good "beyond the borders" areas. They can give a scenario a direction while leaving the players enough options to avoid an outright railroad.
Once the characters are higher level and can start teleporting or using magical air transport, the appeal of islands as setting diminishes as their isolation is diminished, but they still retain a romantic air of "what if" about them where things stranger than the norm, even for a fantasy campaign, can be encountered and interacted with.
"At last they stood on the ultimate pinnacle, their hair stirring in the sea wind. From their feet the cliffs fell away sheerly three or four hundred feet to a narrow tangle of woodlands bordering the beach. Looking southward they saw the whole island lying like a great oval mirror, its bevelled edges sloping down swiftly into a rim of green, except where it broke in the pitch of the cliffs. As far as they could see, on all sides stretched the blue waters, still, placid, fading into dreamy hazes of distance.
"The sea is still," sighed Olivia. "Why should we not take up our journey again?"
Conan, poised like a bronze statue on the cliffs, pointed northward. Straining her eyes, Olivia saw a white fleck that seemed to hang suspended in the aching haze." Robert E. Howard "Shadows in Moonlight"
"Hex 0229 Isle of Greysend-A shipwrecked squadron of charmed knights who have married all of the goblin women" Wilderlands of High Fantasy, Map Two: Barbarian Altanis