How do we decide which greenhouse gases are "good" and which are "bad", and on what levels?

The main mechanism of the greenhouse effect depends on how efficiently the gas in question absorbs near infra-red light from the sun and then re-emits it as far infra-red (heat). From the wikipedia page below. the most important are Water vapor H2O       36 – 72 %  Carbon dioxide CO2   9 – 26 % Methane CH4             4 – 9 %  Ozone O3                   3 – 7 % Lots of other factors affect the overall effect of each gas. http://en.wikipedia.org/wiki/Greenhouse_gas#Impact_of_a_given_gas_on_the_overall_greenhouse_effect

Greenhouse gases are not bad per se; if we did not have greenhouse gases, life wouldn't exist. Greenhouse gases make the earth warm enough for us to live. But too much of a good thing is bad too. Too much greenhouse gases in the atmosphere will heat up a wee bit too much. To take an extreme example, the humongous amounts of greenhouse gases in Venus's atmosphere make it hot enough for lead to melt. We clearly do not want that. Also the atmosphere is highly non-linear. Ice ages have occurred because of tiny wobbles in the earth's orbit. So it is not the case that "a small amount of a gas" has a "small effect". Ozone is a different ballgame altogether. The reason why ozone depletion is bad is because the ozone layer in the stratosphere absorbs UV radiation that would otherwise be harmful. This has nothing to do with global warming, (but everything to do with our surival. Turns out we are a pretty brittle species, by the universe's standards) If one were to give a thumb rule for deciding what is a good thing and what is bad, I guess I would say that I would be wary of anything that significantly changes the chemistry of the earth's atmosphere. Significant here is of course in terms of impact, not in terms of actual amounts.

Greenhouse Gases   Air chemistry: 78.05% nitrogen 20.95% oxygen 0.93% argon 0.039% carbon dioxide 0.00123% neon 0.0005% helium 0.000185% methane 0.000114% krypton 0.000055% hydrogen 0.000006% xenon 0.000005% ozone I find it hard to believe that CO2 which makes up less than 1% of the total greenhouse gases is destroying the planet.  I was sure the amount of CO2 would be much higher than that. Not that I don't think global warming is not happening.  On the contraire, it just amazes me that such a small amount of CO2 could be causing all of these problems.

Ozone depletion over the poles lets in UV which is risk for sunburn, cancer, and cataracts. It is not a problem outside of the poles because sunlight generates new ozone faster. Trying to fight warming by depleting non-polar ozone would be ineffective (ozone is only a small part of the greenhouse effect) and difficult (sunlight generates ozone) as well as increasing the health risks of UV.