1.) a resource patch (iron, copper, oil, etc.)
2.) a coal patch
3.) water access
without mowing down literally dozens of tightly packed spawners and worms to clear large segments of enemy base from your area. Nine times out of ten when you do find a resource patch, it's at least partway underneath a large enemy base and no part of it is even accessible without clearing out most or all of the base. You probably have to travel through one or two bases just to get to the nearest coal patch or water source. It makes it so that far too much time is spent mowing down enemy bases, and far too much science is spent on military upgrades when I'm not even interested in fighting the enemies, just so I can uncover new resource patches to move to as the old ones run out. The alternative is to set resource richness higher, but that just hands me the resources and eliminates the exploration aspect from the game.
Here is my explored map on my current 0.15 game, with rail world settings:
I couldn't find very many forum posts on this topic though I'm sure it has been discussed before. I did find this post.
Another point I'm sure people will bring up is the very popular mod Resource Spawner Overhaul. This goes too far in my opinion, as its default setting makes the enemy bases unattractive and less organic in shape, repeats the same base shapes over and over again, spreads them so far apart you can easily explore and conquer almost any part of the map uninhibited, and the settings are difficult to understand because they're not only in code but work in some arcane fashion that takes a lot of trial and error to understand.
I can say the same about the vanilla map generation: the settings do not function as labeled. When you change an item, it doesn't always change the way it's listed. I remember in the past when I set oil patches to very small and very frequent, I got no oil at all because apparently very small was too small to spawn, and later testing revealed that even small would fail to spawn enough that even at very common you barely found any. Also back when terrain segmentation was an option, I never got around to testing it much but I set terrain segmentation to very low on one map and traveled 4000 tiles before the map showed a single change in biome type.