The perfect world for humans is and probably will remain Earth. New planets and moons, once they are terraformed, will probably tend to degrade back to their previous form or at least to a place where human life will be hard, if not impossible.
Atmospheric Losses Edit
Many celestial bodies proposed for terraforming are too small to maintain the needed atmosphere. Also, many of them lack an internal dynamo to keep a strong magnetic field, to protect them from solar wind erosion. Depending on size, in some cases changes will occur longer then a human lifetime, but there are places where even in a few decades changes will be visible.
Not Enough Mass Edit
Some celestial bodies, like Luna, Mercury, the Jovian moons, the icy moons of Saturn (except Titan, Triton and the large Kuiper Belt Objects don't have enough mass to keep a large atmosphere in place. After terraforming, they will have a very large, fluffy atmosphere. They will lose selectively the most light gasses. Even on Earth, light gasses like hydrogen and helium are lost into space. Since a breathable atmosphere contains nitrogen and oxygen and both gasses have similar mass, the loss will be of both in almost equal quantities. Oxygen can be produced locally, from the existing water. After terraforming, nitrogen should be found almost completely in the atmosphere, like on Earth. Supplying nitrogen will require to transport it from other places. Still, life is possible with a rarefied atmosphere, that contains only enough oxygen. Cosmonauts breath an air which is 3 times more rarefied then on Earth, but composed almost completely of oxygen. Nitrogen is still vital for plants (and also for animals) for the creation of proteins.
Solar Wind Erosion Edit
The solar wind can dramatically affect a planet that has not a strong magnetic field. This is highly visible for Venus and Mars. Solar wind erosion is selective. It doesn't blow away all gasses on the same rate. It brakes water molecules into hydrogen and oxygen. While oxygen is heavier and remains in the atmosphere, hydrogen is much lighter and escapes fast. The stronger the radiation, the faster water will be removed. So, Mercury will be more affected then Mars. If we talk about the icy moons of Jupiter and Saturn, the effect is too small to be sensed over millennia. Other solar systems have mush stronger solar winds (for example B - type stars).
Activating a planetary dynamo is out of question, it is too expensive and might have catastrophic consequences on a planet (including quakes and volcanism). The most reliable technology is to replenish water by diverting icy Kuiper Belt Objects. However, these bodies have a different type of water then Earth does. They contain water with different amounts of deuterium, as well as many types of salts. Nobody wants to have a too salty ocean. Choosing the right comet to divert is very important. Also, it is important to smash comets into smaller meteorites, to avoid producing a catastrophic impact.
Temperature Corrections Edit
For terraforming, an Outer Planet needs greenhouse gasses, while an Inner Planet will need shielding from excess light and heat. As life conquers the new world and then as colonization progresses, global temperature will change. It is very important to adjust the heat, in order to avoid a runaway greenhouse effect or a runaway ice age. On the other hand, over time, greenhouse gasses might be decomposed by ultraviolet light, by lightning or human activity or even they might get absorbed into rocks. The same, radiation shields of an inner planet might lose efficiency. An orbital shield of mirrors and lens will change shape over time. All objects will tend to group into a ring, covering less surface. If we have an atmospheric shield, made by various gasses, then it might lose efficiency as these gasses are destroyed by radiation, lightning or other factors.
It will be very important for future settlers to continue production of greenhouse gasses or atmospheric shields whenever it is needed.
A terraformed planet will always keep the fingerprint of what it was before. On Venus, after terraforming, there will still be acids at some depth underground. Water erosion and landslides can expose them. Other planets might also contain toxins at some depth. In some cases, during terraforming process, scientists decided to add other chemicals to neutralize existing toxins in the ground or in oceans, creating macromolecules and complex ions. But after some time, the process starts to reverse and the new colonies will have to repeat the process.
After terraforming, water will start to enter deep underground, bringing salts and even heavy metals into rivers. On Earth, water has been flowing for long enough to clean majority of ground waters. On a new terraformed planet, settlers will not have time to wait millennia for nature to reach a new equilibrium.
If the oceans are too salt or contain a dangerous substance, then a solution would be to sacrifice a sea, to deposit part of the salts there, covered by a thin layer of water, so they won't be blown away by the wind.
Water & Climate Edit
Water and climate are strongly connected, so they will be listed together.
After terraforming, climate will need time to get into an equilibrium state. Hurricanes, droughts and floods will be common. It is very hard to predict what the climate will be after this first phase ends. It all starts with water movement. Ocean salinity, formation of ice caps, evaporation on seas surrounded by desert, they all have a significant impact on climate. On a planet that does not have connected oceans, but isolated seas, over time, water will move from one basin to another, affecting climate.
Some large scale engineering projects will be needed, in order to keep the water and climate equilibrium. They might include diversion of large rivers, construction of large canals between seas, breaking mountain ranges or the construction of artificial mountains to block certain air and water currents and not only.
We don't know if such large scale projects will be possible for the new colonies. Maybe they won't have the resources. Still, they will have to do something on a lower scale. They will have to protect themselves from floods, to irrigate crops (if needed), to produce energy and to keep a transportation infrastructure functional.
Population Pressure Edit
Overpopulation and extensive industrial development can have a huge pressure on the newly formed ecosystems. See Population Limit for more details.
Despite of this, there are many other dangers, some of them still unknown. Solar flares, solar explosions, meteorite impacts, they all can affect a terraformed world. Some can be avoided, some cannot. In some cases, we can do something to prevent from happening, in others, all we can do is protect ourselves from the inevitable.