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Some of the points are easily observable in both our and extra solar systems. For example:

"For suns to spawn Earth-like planets they must have sufficient “metallicity,” which is necessary for the formation of terrestrials rather than gaseous planets."

This has been observed in extra solar star forming regions.

"Earth is in a “habitable zone” within the galaxy as well. Closer to the center of galaxies, radiation and the presence of wandering planetoids make life there unlikely."

Fair point, but if you create a "donut shape" around the center of the Milky Way, delineating this habitable zone, there are lots and lots of stars to consider.

"Earth’s orbit around the sun is an almost perfect circle rather than the more common “eccentric” (elongated) ellipse."

The orbits of Venus and Mars are also highly circular. So are the orbits of most of the others in our solar system. So, just in our neck of the woods, we are something like 6 out of 8.

"Earth’s axis is tilted just enough relative to its orbital plane to allow seasonal variations that help complex life but not so tilted as to make those variations too extreme."

Mars is currently tilted at the nearly the identical angle. Again, just in our Solar System, we are 2 for 8.

"Earth exists in a disk-shaped spiral galaxy (the Milky Way) rather than in an elliptical (spheroid) galaxy. Spiral galaxies are thought to be the only type capable of supporting life."

No reason is given for this assumption. If a DISK shaped galaxy has a donut shaped habitable zone, then a SPHERE shaped galaxy should have a Spherical Shell shapped habitable zone.

"Our sun is just the right kind of star, putting out a fairly steady amount of energy. Other types of stars are more variable in their output and this variance can utterly destroy life or cause it to be unsustainable due to the extremes caused."

Red Dwarfs also fall into the category of steady producers of energy, and they have the added advantage of being much, MUCH longer lived.

Just a few things to consider when pondering how rare the Earth is. Also, keep in mind what a tiny, tiny, really, really tiny fraction of the galaxy we have had any real kind of look at.