a long history tends to kill off many monsters. Yes they reproduce, but usually more dragons (for example) get killed than are born. Longer histories have fewer megabeasts. Forgotten Beasts and titans are usually mostly still there as they are almost immune to being killed in worldgen.
Civilization will have spread more, meaning more towns, cities, mountainhomes and whatnot, so slightly less choice on where to embark. On the other hand, there is also more chance of civilizations dying out.
Migrants that show up will most usually have and bring their family. They'll also have more skills and a greater variety. A long history has more chances for war, so you have a good chance to get skilled soldiers if that happened.
Necromancers need time. First they have to grow up and start to fear death, then discover the secret and then get enough zombies to make their towers. A longer history means more necromancers.
Were-beasts tend to get out of hand sometimes in long histories, converting entire towns to their 'clan' and making lairs all around. Vampires also tend to accumulate as they rarely die and people still get cursed by the gods as time passes in worldgen
There also has been less time for erosion, which affects rivers and mountains
so in short history:
more megabeasts, fewer (historical) migrants as civilizations havn't had time to breed and spread. Lots of available land. Nearly no necromancers, fewer werebeasts and vampires. Migrant skills mostly farming and such related, except for the pregenerated exceptions - no cities yet, no fighting
I'm probably wrong somewhere and might have missed some points, but that's the general thing