He says he has lots of differences with Hillary, but the tenor
of the attacks on her bother him, and he wants to save ammo for the
Republicans. Yes, he's angling for a spot in the second Clinton
administration.
UPDATE: Jennifer suggests below that he's angling for VP. Keep
in mind that even if he doesn't get the veep slot, Richardson will
still be in the running for some other position in the Clinton
administration (perhaps Secretary of State), so he has that much
more incentive to pull punches.