Mountains are another important and definitive factor. They affect the world in numerous ways, from the basics of the world--like weather patterns--to the cultures of the world--changing travel patterns and accounting for resources.
Mountains form in four distinct ways: there are folded mountains (the most common type of mountain), volcanic mountains, erosional mountains and fault-block mountains. How a mountain range forms is important to the type of environment that will be created and what type of rocks and minerals, will be found there.
Folded mountains occur when tectonic plates push against on another. To relieve the pressure, the plates are pushed upward and form mountains. This is also true of the mountains formed when two continents collide. The force of the collision can force the rock between them--which used to be sea floor--to squeeze upward, forming a mountain chain along the line of the collision. This is how the Himalayas formed, for instance, as well as the Rocky Mountains.
Volcanic mountains form as a way of releasing internal heat and pressure. They can form in chains or as individual mountains. In fact, most isolated mountains are volcanic. The Hawaiian volcanoes are great examples. Laccoliths are also an effect of volcanic activity, but these dome-shaped mountains form when magma fills a pocket between layers of rock, causing the upper layer to heave up. The magma inside cools and hardens. Bear Butte is South Dakota is an example of a Laccolith.
Erosional mountains are created when the surrounding land is weaker than a given pocket of stronger material. The weaker stone is eroded away by wind or water, leaving tougher rock to stand alone. These, too, can form solitary mountains. Some beautiful and wonderful examples are the erosional mountains of the Stone Forest in Hunan, China.
Fault-block mountains are similar to folded mountains, in that they are caused by tectonic movement. As tectonic plate move along fault lines, rubbing against one another, sometimes pieces break off and are pushed up when the tectonic plates move beneath them. Basically, fault-block mountains, as I understand it, are the rubble left behind by tectonic shifting. They're characterized by sheer cliffs. The Sierra Nevadas are an example of fault-block mountains.
Mountains can change weather patterns, having either an "orographic" effect or a "rain shadow" effect. On the windward side of mountains (the side against which wind blows) you get the orographic effect, in which there is dramatically higher rainfall. On the leeward side, you get the "rain shadow" effect. The mountains block rain and moisture-bearing wind, or force storms to peter out before they can cross the range. Land which falls in this "rain shadow" can be dry, even to the point of becoming a desert. Such as the Atacama Desert in Chile, one of the driest places on earth.
Mountains also create handy borders. While some will be navigable, others will not--or will be only by those who know them well. Nations often declare their borders along a range of mountains. Mountains limit agriculture as well, while it's absolutely possible to farm in the mountains, the form of the mountains present challenges. How your cultures deal with those challenges contributes to their tech, their philosophies, their religions, their agricultural cycles, calendars, and any number of other things. Even not dealing with these challenges (i.e. cultures which are non-agrarian) shapes the culture.
Terraces carved into the mountains not only provide land for farming, but can provide structure for buildings and cities higher up in the mountains. In any place where there is a lot of rain, making sure your mountain top, highly defensible city doesn't get washed away is very important. Machu Picchu is a glorious example of what people can create on a mountain top, even without a wheel and axle, or even iron tools. The terraces of Machu Picchu not only provide structure, but were engineered to drain rain water away from the city.
And if that doesn't fire up your worldbuilding juices, well I just don't know what will! So tell me. What gets you in the worldbuilding mood?