I know I'm probably WAAAY off on this, but I couldn't help but think that this last half of Series 7 is all going to be centered around the Doctor's past. We have 2 episodes with the GI as the nemesis. We know there are Ice Warriors, & Cybermen in the near future. We're getting Ten, Rose, Zygons, & possibly a Dalek in the 50th.
For me though, last nights episode seemed like there was a lot buried right beneath the surface. We have the reference to Susan right off the bat, & then the entire episode has a 'bad guy' that is basically considered a god, that they refer to as Grandfather. He feeds on the memories & stories, & then the Doctor believes that all we are is made up of our memories & experiences.
It's hard to put exactly what I was thinking into a coherent post, so I'll just leave it at this-
It seem s like there were a lot of parallels between the Doctor & Grandfather, like how he is so old & jaded to the universe, the only way he really has joy is by experiencing the universe through the new experiences of his companions. They both need the experiences, & in a way, feed on them. The Doctor has expressed his joy when both Clara & Amy joined him, because of the new eyes in which he could see the universe, & Grandfather needed 'pure' or new stories or else he'd grow violent & consume everything. Since Who came back, it's been hammered into our heads that the Doctor can not be left alone or else bad things happen.
Hope that's clear enough:behave.