Here are the following facts that, I believe, make our little rock conducive to life as a human.
The transparency of EM radiation through Earth's atmosphere
It is estimated that we get 80% of our information about the world around us from our eyes. Our eyes use photons to see. Where do most of those photons come from? The sun. Good ol' sun, churning out a bazillions of photons per second, all for the benefit of bouncing them off things and into my eyes. Oh, and apparently plants use photosynthesis. Yeah, like that ever helped anyone.
Our molten core
From all that we know of planet formation, the center of the Earth should be solid by now. It is, after all, about five billion years old. That's more than enough time for it to chill out, relax, and stop squishing about. The thing is that we got a lot of radioactive crap down there. As these atoms decay oh-so-slowly, they release energy. That stupid energy keeps all that molten gunk glooping around all day long. As our sloshy core mushes around, it happens to create a magnetic field around our planet (check out Van Allen belt, not Van Halen belts). This field sends all of the high-energy, mutation-inducing solar wind (particles of matter ejected form the sun) to our uninhabited, f'in cold poles (and makes pretty colours).
Tilt to the ecliptic
The Earth goes around the sun. You knew that. Well, the plane formed by the Earth going around the sun is called the ecliptic plane. Earth is tilted from that by ~23.5 degrees. This gives us seasons. While most anthropologists agree that early human life came out of the savannah, a nice temperate environment, we would never have invented hockey if it was always so nice out and thus early man would have committed suicide from sheer boredom. And seasons also happen to play a role in plant cycles. Plants! Sheesh! I'm tired of them and their CO2-to-O2 conversion. La dee da! Look at me, I'm so hot... Screw you, atmosphere!
Ozone (O3) absorbs UV rays (another healthy source of DNA-damage) by getting broken up into O + O2. It also absorbed the 80's trend of giant hair fueled by hairspray.
The Earth happens to be 1 Astronomical Unit away from the sun! Lucky for us because that seems to be just the right distance to support human life. Venus is hot enough to melt lead at its surface and Mars is cold enough to pop Arnold Schwartzenegger's eyes out. Also, amino acids and other proteins seem to find it just right between -50 and +50 (though they tend to stay near to the 0 to +20 regions).
24 hour rotation
I'm not sure how yet but I bet plants love an alternating day and night. Note: Just as the moon always has the same face towards us, the Earth is slowly losing rotational energy and will eventually have the same face towards the sun. Of course, the sun, in response, will explode.
Lots of water
Life apprently loves the crazy chemical that is H2O. If you didn't know that water was useful until I just said it, well, you're an alien and I just outed you. In your face, xeno!
Checklist of naturally occurring elements from the periodic table
I don't know who did the collecting but someone definitely had "gotta catch 'em all" fever. And they did. We gots 'em all from 1 (Hydrogen) to 92 (Uranium), except 43 and 61 which have no stable isotopes, in abundance. With this variety of tools to play with, I can't wait to see the silicon-based life that you know is coming. Not just any old system or planet gets all these goodies. In The Beginning there was only Hydrogen. It takes a lot of Hydrogen before you can get hot enough to make Helium (2). And anything above Iron (26) requires a supernova. Seriously. Do you know how long it takes for anything to go supernova? I'm still waiting for Star Xb-7465-21 to go (I gots money on it). So looking around at all the freakin' atoms with more than 26 protons makes me think, hot day-um! That's a lot of kablooie...
That's all for now, if I think of anything else, I'll post later...