Drawing your power from a demonic source seems like a good place to start. But my point was that there doesn't have to be a consistent dividing line, since it would be entirely in character for Dean to judge other people's spellcasting more negatively than his own. The whole "partners with Crowley" thing underlined that a few times.
It's not up to you if Cas comes back
Seriously. I already typed out the "Does anyone have a link to where Misha said sincerely he wanted more work?" reply to a related tumblr post, but I did end up hitting cancel on that one. But, seriously--why is his employ their problem?
Then again, I refuse to believe the person who said it was sad that there were Dean fans and Cas fans, but so few Sam fans (or insert actors instead--it was blurry). That really has to be the result of isolating yourself in fandom. I refuse to believe that Sam (and Jared) has fewer fans than Cas, whether you count people who quit watching (aka the D/C or Casgirls who thought S6 ruined him and their ship) or just the current watchers.