Also the bi-generation seemed to be just "because", no real explanation of why it happened based on what was going on. No real thematics for why this regeneration was different.
Ironically give that you are bemoaning the lack of cleverness, I think is this an example of something that may have been too clever, which made you miss it. RTD (and indeed Moffat) like to bury plot points in fast conversations.
In the previous episode the Doctor invoked a myth (the salt) at the edge of the universe where the boundaries were thin. This let myths into the universe.
The Toymaker was the first to cross into the universe. The Doctor realised what he’d done and mentioned it in the episode -- "Because I got clever, didn't I? I cast that salt at the edge of the universe. I played a game and let him in."
Bigeneration was an old time lord myth. “Bi-generation. I have bi-generated! There's no such thing. Bi-generation is supposed to be a myth, but… look at me.” Never happened before. Isn't supposed to be possible. Came true.
Goblins are a myth (they even call it out in the song -- "He’s not a myth, he’s an actual thing!"). Came true.
The theme for this season is that myths are coming true. Prompted by the Doctor’s actions in Wild Blue Yonder.
You have to listen for it but it’s all there and clearly explained.
(It’s also part of RTD’s desire to add more fantastical elements to the show, which is why he’s added this breakdown of the sci-fi/fantasy barrier).
I'm not sure how I feel about this yet. I like Who at least pretending to be sci-fi and I’m not sure I want it to become fantasy. But it is all there.