I think this is one of the most beautiful things in the special.
This is not even on the meta “Oh my God, it’s Tom Baker!” thing. Not really. It’s about the hope this gives for the Doctor.
The Curator is, IMHO, the last incarnation of the Doctor. He’s done all he can, seen all he can see. Now he’s Sherlock Holmes as a Bee Keeper. Instead of retiring from the action-packed life to being, like, the President of the restored Galifrey, he’s retired to Earth— his adopted home.
This is one last trip for him. It’s an extended version of the last twenty minutes of Journey’s End for him. Just think about how long he’s been in this form. He most likely regenerated into Tom Baker from Planet of the Spiders. So how long has he lived? Eleven has lived for two hundred years in his current incarnation without any real visible change in appearance. It’s possible the Curator has spent millennia in this incarnation.
And he’s a Curator. He collects things from the past. He remembers. He’s perfectly content not dashing about any longer. I like to think he watches the things his past self does, recalling the good times. He looks in on his companions in their own times. Since he’s on the slow path now himself, he can visit the Ponds without risking the destruction of the timeline. He and Jack and Barbara and Ian can meet up and laugh about watching the world pass them by very, very slowly.
The Curator is the Doctor’s reward for all he’s done to keep the universe safe.