Robot Number V said:
*sigh*
It started out good, but then (much like the first episode, actually) it devolved into poorly explained nonsense and the Doctor did something that SHOULD have had enormous ramifications but inexplicably didn't. (If you're curious, I'm referring to the fact that in the first episode, the Doctor murders literally hundreds of people and doesn't even feel bad about it, and then allows the thing that forced him to murder hundreds of people to escape without even making the slightest attempt to find out what it was)
Look, I'm willing to overlook the absolute pointlessness of the monster in the glass cube. I'm willing to overlook a random song somehow keeping a giant psychic parasite asleep. I'm even OK with the song inexplicably ceasing to work for no apparent reason. But some things about this episode reeeeeally stretch my tolerance for bullshit.
In this case, Clara defeating the Psychic Vampire with some multiverse-style nonsense. If the leaf represents not only her memories of what happened, but also every possibility that didn't happen (no idea how that works, by the way) then doesn't the same principle apply to literally every other memory the thing has absorbed? What makes the leaf so special?
And also, why is it not a big deal that seven planets no longer have a sun? It even shows that everything has gone dark. Seriously, that is fucking GAPING plot hole.
I was a lot happier when it looked like the Doctor would defeat it just by letting it read his mind knowing it would explode from too much unadulterated Badass. THAT makes enough sense to me for me to accept it.
I know Dr. Who has always been ridiculous, but it's always had some kind of internal consistency. If they were going to have a platoon of space rhinos raiding a hospital on the moon, the actually took the time to explain how that happens. Nowadays, it seems like the writers just say "It's Doctor Who, it's never made any sense! Just throw in a speech about the HUMAN SPIRIT and we're good!"
You've seem to have missed a few things that may clear up some the confusion.
1. It wasn't a sun and it was never referred to as the sun within that system. The Doctor explicitly states that it is a planet, and that the religion says it is the planet from which all life in the universe originates.
2. The Doctor also implicitly states that the song doesn't actually do anything. Hence why the girl was not at fault for getting the song "wrong". It just so happened to be the time in that beings life cycle to feed.
3. The leaf thing can be interpreted several ways. The most straightforward way is that the leaf had the psychic imprint from her and her father about both the memories of Clara's mother, but also the possible life her mother could have lead. Clara and her father obsessed over the leaf and that possible life in their grief, and thus there was an infinite amount of "stories" stored in it. Memories don't have a psychic imprint, and the creature never tried to absorb an object with that type of psychic imprint before.