http://www.englishpage.com/prepositions/verb_preposition.html
Agreed under letter A
Comment under letter C

I've already discussed that matter with you.  (you discuss something WITH somebody)
We discussed that issue here a few days ago.
We have briefly discussed that DNA is the genetic molecule of life.
We discussed about that one/that case/that issue

   A law, conceived by Republican governor Scott Walker, freezes contracts and cuts salaries and, moreover, it seriously limits the presence of trade unions in the public sector.  We discussed ABOUT that with professor Joseph McCartin, director of Kalmanovitz Initiative for Labor and the Working Poor at Georgetown University, a Washington based research centre engaged in the promotion of labor and the poor.

We already discussed ON THAT...often it comes togheter some more ON THAT page, ON THAT site, ON THAT day, etc.  for example, "Reiser says religion has never been discussed "on that show and probably never will be," ...

   Obs: ON and ABOUT has the same meaning here, but ON has an academical nuance to it, ON is used to talk about serious studies/subjects etc.   ABOUT is more informal.

   Let´s wait and see more comments.

   PP.