??so the coadjoint parabolic subalgebra is somewhat more "photogenic" in the root system picture than the other parabolics are, because its "boundary hyperplane" is orthogonal to the highest weight of the coadjoint irrep itself rather than to that of some other irrep; and this extra photogenicness is essentially the manifestation of the invariant contact distribution on the coadjoint partial flag variety, coming from the symplectic nature of coadjoint orbits... ??or something like that...

but to what extent, or when, do parabolic subalgebras generally have such "boundary hyperplanes" ?? perhaps maximal parabolics always do, with the boundary orthogonal to a highest weight lying on an extreme ray of the weyl chamber. but the coadjoint parabolic isn't generally maximal. hmm, and i guess that in fact there's really only an unambiguous "boundary" hyperplane when the parabolic is maximal. nevertheless the coadjoint parabolic has a special pseudo-boundary hyperplane, namely the mirror orthogonal to a long root.

(does the mirror orthogonal to a short root also bound a parabolic, and is there anything specially interesting about that parabolic??)