I see people say that too much CO2 and SO2 kill humans. Ok. But what about alien life, the native life that live in these worlds? Aren't they adapted to such gases and other substances that are toxic to us? What is the most imporant thing? An extrasolar planet that we can live or simply the existence of extrasolar planets capable of develop complex life, even being toxic to us? Maybe many planets out there, in real life, will have such toxic gases in higher amounts than in our atmosphere. We are adapted to live in our kind of atmosphere. And aliens are adapted to live in other atmospheres.
Venus and Mars both have high concentrations of CO2. Earth and Titan, N2. Gas giants, H and He.
Criogenic rocky or icy planets (-210°C to -185°C) will tend to have more H, CH4, CO and even N2 in form of clouds. If the planet is small (like pluto or triton), it will only have a thin atmosphere of nitrogen and methane. If it's big enough it may have a thick hydrogen rich air, like gas giants, and if "warm" enough, will allow seas of liquid nitrogen with vast clouds of it. It will look like a snowball Earth!
Cold rocky or icy planets (-185°C to -15°C) will tend to have more N2, NH3, CH4, H or even CO or SH2 (if have much volcanic activity, like Io).
Temperate and tropical earthlike planets (-15°C to 100°C) will tend to have more N2, CO2, O2, H2O, NH3 and N2O (if the air pressure is high enough to keep ammonia liquid), SO2 or SH2 (if they have much volcanic activity). Or CH4 and other hydrocarbons and organics (if it's a carbon planet).
Hot rocky planets (100°C to 500°C) will tend to have more N2, CO2, O2 (maybe), SO2 and Na (if it's hot enough).
Extreme hot rocky planets (500°C to beyond) may have Na and even silica or iron vapor!
Planets that have high CO2 and SO2 are planets that have or had much volcanism and kept such gases. Planets with lots of N2 and O2, like Earth, had much ammonia and water transformed by organisms, ultraviolet and storms. Some more exotic planets may have higher amounts of Cl2, F2 and Br2. And gas giants will have much H2 and He because... they are mostly made of light gases.
First example: A hot rocky planet with sulfuric acid seas and clouds, with it's own biosphere with complex life, probably will have an atmosphere made mostly of N2, O2, SO2 (instead of CO2), H2O and H2SO4, and maybe will have Cl2 too, if it's halogen rich. Halogens in this planet will play a vital role in native life, such as hydrogen do on Earth. And also metals like iron and maybe even silicon!
Second example: A cold rocky planet with vast seas of ammonia, also with complex life, probably will have an atmosphere mostly made of N2, CH4 (instead of CO2), NH3 (instead of water), PH3, N2O and C2H2 (it would be their "oxygen") and some more hydrocarbons. But if it has seas of water mixed with ammonia, maybe will also have a lower amount of O2 and H2O. Metals like sodium, magnesium, calcium, rubidium, potassium and maybe cesium, and including boron, will be important to generate electricity that will be stored in cells like batteries, because life on colder planets will have to rely more on electricity, and because ammonia with water dissolve well these metals, electricity might be a viable source of energy for life.
Third example: A temperate rocky world with vast seas of hydrocarbons, such as gasoline. The vast seas of gasoline and it's own biosphere maintain an atmosphere mostly made of N2, C2H2 (their "oxygen"), ethane and propane (their other "oxygen"), methane obviously, maybe also butane, simple and complex organics derived from the reactions in the atmosphere, benzene, and CO or even SO derived from volcanism. It is said that more complex hydrocarbons are better for life than simple hydrocarbons, so a hot or temperate carbon planet might be better than a cold one.
Fourth example: A cold rocky world with vast seas of alcohol or formaldehyde, also with life. The air would be mostly N2, CH4 with minor amounts of O2 derived from reactions. But the oxygen will have to stay low, keeping a reducing atmosphere, so the oceans won't catch fire!
Anyway, planets "identical" to Earth might be more an exception than a role in the universe.