How does earth's atmosphere differ from the atmosphere of other planets?
Abundance of oxygen
The main difference between the Earth's atmosphere and the one of other planets (such as Venus and Mars) is that about 21% of it is composed by oxygen.
Oxygen is a key element to sustain any complex life form on the planet. Its origin is due to the presence of vegetation on the land masses and of phytoplankton in the oceans. Both produce oxygen by means of photosynthesis.
Oxygen plays also a role in weathering thanks its oxydative capacity.
Our atmosphere is also dense enough to generate pressure at the surface and to act as a thermal blanket.
Another unique characteristic of our planet is that it is at the right distance from the sun (not too close or too far) allowing for the presence of water in liquid form and thus maintaining a water cycle of which the atmosphere is a great part whit the clouds and rain.