Well... there are two variations of the fine tuning argument that I know of.
Fine tuning of measurable quantities
Argument:
The size of the earth, the distance from the earth to the sun, etc are all necessary for life to exist. Change the distance to the sun, and life could not be sustained.
It would seem that everything in our solar system is fine tuned to allow life to exist on this planet.
But
how fine tuned are these quantities really? If the size of the earth was changed ever so slightly, would the earth not be able to sustain life
of any kind? If the earth was just a little bit closer to the sun, would it be too hot for life
of any kind to exist?
We know that the evolutionary process fine tunes life to the quantities (and not vice versa), so it would seem that quantities do not need to be
very fine tuned in an evolutionary framework.
In this vast universe, is it really so unlikely that one planet with these particular quantities all satisfied (within certain limits) would exist?
Fine tuning of the laws of physics
Argument:
The universe in itself is fine tuned to support life. If you change certain universal constants or laws of physics even a little life can not exist.
But
it would seem to me that we can't really know what would happen if the constants of the universe where different. Maybe atomic structures we see today could not exist under certain configurations, but do we know that such structures are really necessary for life
of any kind to exist (certainly it is for
our type of life, but what about other types)?
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