The places of the imagination must have some semblance to earth, or we are confused by the nonsense as we try to take in an alternate world. Usually there is one difference—one factor that is altered to set the ball rolling—resulting in a cascade of other changes.
With this idea in mind, I followed three examples of that might seem like a minor change to life on earth, yet if it were to occur it would create an unrecognizable environment or an unrecognizable humanity. The first would be to eliminate paper–that ubiquitous thin tissue made of some organic substance (usually plant matter) which allows us to make printed books and money. Would you have created a society in which everyone carries a clay coated ceramic tablet, rather than a day planner, as in early Mesopotamian civilization?
The difficulty with eliminating paper is that it can be a made of most kinds of plant matter. Papyrus reed was an easy starter to make paper, as was wood bark, but so were the fibrous plants used for fabric like cotton and linen. Multiple cultures developed their own type of paper individually. The planet without paper would be one that is without any plants more complex than moss. The land would be either rock, frozen artic or a desert. And, that would be a drastic change. You suddenly have a world like the uninhabitable zones of earth.
On the other hand, you could get rid of humanity’s desire to record anything. Imagine a world full of people who do not care to keep to information that is any greater than they can hold in their head. That change seems just about as drastic as eliminating all of the vascular plants.
Let’s imagine a world without guns, like the Hawaiian Island before Captain Cooke stepped on shore. The islanders venerated Cooke on his first arrival with the metal sticks that could kill from afar. And, when he returned, they killed him. Before the Europeans arrived, the Hawaiians fought with sharpened sticks. Their skirmishes were frequent but resulted in few causalities. No chieftain managed to control all of this archipelago with this limited power. Even would out guns there would be fighting and killing, just not of an efficient nature.
If the gunless society is to be more advanced, the lack of a few resources could eliminate the development of guns. One is iron and tin which makes steel. However, steel is the backbone of most of modern buildings and vehicles, so the material for structures and infrastructures must be imagined also. Developing anything like an industrialized society would have to take a completely different route. The other option would be to eliminate gunpowder and removing Francis Bacon from history is not enough. The Chinese also discover the explosive qualities of nitrates. This leaves the choice of removing a key element (like sulfur) or creating a society that abhors chemistry. That would strip the world of most advances, and not just remove the ability to shoot guns.
The change in the third imaginary world seems a bit innocuous at first. How different would a world without glass be from ours. No fake jewels, no jars, and windows would be made out of flimsy paper. You may not realize it, but glass was being produced by numerous societies in the late bronze age. Melt sand and you have glass, as sand is composed of mostly silicon dioxide. Creating the heat necessary to make glass was the major challenge, but obviously not too great of one. Your major challenge in creating a world without glass would then be creating a world without silicon. Just get rid of all the sand.
World building can be a Sisyphean task.