Minecraft wiki entry for Old, associated text: With this world type, the Overworld was the only dimension that generates. Attempting to go to other dimensions resulted in the portal not being activated.
Villages sometimes generated near the world border and get cut off by it, resulting in a village without any meeting point, only one house, or a jobsite building, or even just a road.
Lakes could still generate normally, but lava lakes do not generate near bedrock. Also, lakes did not generate under Y=10, due to the ground being completely solid.
Desert wells rarely generated in this world type.
On rare occasions, buried treasure and fossils could generate.
New Mountains still generated with experimental gameplay enabled, but are cut off at Y=127, causing the top of them to become a flat plateau of stone. This usually happened if mountains generate past the meadow layer.