That whole end scene is on my mind. I can't help it. I keep thinking of three possibilities. (A) Dean is in Hell (B) Dean is in limbo because Sam or someone else intervened and messed up the going to Hell process (C) Dean is in his own personal Hell where one of his worst fears of being alone have been realized.
I've rewatched the end scenes a few times - I'm sad - and I just don't know. That just doesn't seem like Hell to me. So I keep on leaning between Dean being in some limbo or trapped in his head because something (or someone, like say, Sam) intervened. If it's that option, holy crap, Sam! But it could also be representative Hell. I've heard of beliefs where Hell is what you make it and that could fit here too. I keep swaying toward the middle option though. That seems the most complex and full of possibilities. I guess we'll see though.
But dude. He had hooks in him. HOOKS.
*flails some more*