I got lots of great comments under my recent blog post describing a world in which the day and night both last 100 years. I have been meaning to collate some of the ideas I especially liked, so here are some of them:
1) The climate would be changing all the time. This seems a simple observation, but this would be a world of constant climatic churn, because weather patterns would always be shifting. The landscape in a certain spot could change quite noticeably even over the course of a decade.
2) Human civilizations would be largely migratory over the course of the day, perhaps even in the form of moving caravan-cities, but this would bring about quite interesting effects. First, there may be oceans, which would have to be sailed across on annual migrations.
Second, there would be "bottlenecks" formed by mountains, deserts and so forth. In such places there might be permanent settlements effectively charging tolls to pass - maybe operated by a different race equally capable of living in the day and the night (maybe run by dwarves or gnomes or something). These bottlenecks with permanent settlements would also get rich from trade.
Third, migrating peoples would follow regular pathways. While only the very oldest people might remember, it would be part of oral (or written) history. This would mean that people would bury or otherwise hide caches of supplies in regular spots to pick up 100 years later on the next passage. Some of these regular pathways might cut across the poles because that would be a shorter route.
Fourth, migration might go in reverse. A viable tactic for a civilization could be to stay put for 100 years until the night arrives, and then up sticks and travel across the night, circumnavigating the world, to find the day again as it advances from the other direction. This would be very dangerous because the night is full of dangers, but might also have big benefits once you were back in the day time. Nomadic peoples might also do this if there was a natural barrier - they might go from East to West over the course of the day until they come up against a wall of mountains or an ocean, and then wait there until night to fall, before quickly making a dash across the night back to day time on the other side.
Fifth, following on from that, some people might be cannibalistic or semi-cannibalistic. During the day they travel from East to West, and then turn around and go back across the night, living off the meat of designated tribe members on that journey.
Sixth, on the oceans there would be water-going people who would be, to some extent, much freer - but who would be in real difficulties in the ended up accidentally getting iced-in if they were in the wrong place at night.
3) Another tactic for civilizations may simply be to stay put. During the day, vast food stores could be gathered and huge underground networks dug to store it all. Then during the night everybody would move below the surface to effectively hibernate.
These underground stores would be easy targets for more nomadic types, especially as night approached. They would have to be very carefully defended and would make a great excuse for creating dungeons for PCs to explore (especially where for mysterious circumstances the owners have left, or have all been killed?).
The digger-types would be obsessed with record keeping and cycles. A nice tweak was the idea that, in this world, there might be regular changes in wind direction associated with different phases of the day and night. Digger-types might have special minarets built above their cities and angled specially to make sounds or vibrations when the wind changed around the time of morning.
And some of the more extreme diggers may abandon surface living entirely and descend further and further below...
4) There is also, of course, MAGIC. Perhaps some particularly advanced societies would have magical means of surviving the night (or day) - or maybe even travelling around in floating cities.
Putting this all together, I having been thinking today about a campaign setting taking place in The City Standing Like a Candle in the Night - a walled fortress inhabited by a great and advanced civilizations able to magically last the night. The PCs would begin there, at the very bottom rung of the social order, early in the night - just a year or two in. Surrounding them would be all kinds of opportunities for adventure. Intrigue in the City itself. Raiding nearby "digger-type" settlements with huge underground caches of valuables (perhaps one of which is abandoned and forms, essentially, a megadungeon, with many of the defences still intact). Stealing from dwarf or gnome toll-takers at a nearby migratory choke-point. Searching for caches left by migratory peoples. Tangling with orcs and other night creatures. Searching for hermits or elder beings who do not move with the regular cycles? Trying to track an infamous Laputa-like floating castle?