The Wikipedia article on electron shells states this, which I (a chemical layman) also always assumed:
The electrons in the outermost occupied shell (or shells) determine the chemical properties of the atom; it is called the valence shell.
It appears this is wrong. What really appears to determine the chemical properties is the outermost occupied subshell, ie. the things called 2s, 3p, etc. (Let's call this the "valence subshell" for now).
In particular, the noble gasses don't have a full valence shell at all (except helium and neon), but they do have a full valence subshell.
The same article also states that
A nonmetal atom tends to attract additional valence electrons to attain a full valence shell.
By this logic, noble gasses other than helium should be reactive.
I suspect this isn't simply a screw-up on Wikipedia's part, since the term "valence shell" is popular, while even the term "subshell" itself is rarely used. "Valence subshell" does return google results, but extremely few.
So, what's going on here?