Storytelling by Steven Moffat:

#1 Introduce the idea of the Impossible Girl in the first episode
#2 Totally forget the concept, except a few passing mentions here-and-there
#3 Bring back the idea, like it's the greatest thing ever and build the finale around it
#4 Make the finale totally devoid of any emotions, suspense or wit
#5 Sit back like you just wrote TV history

Damn Moffat, I was sooooo invested in this character, right?

/sarcasm

If this is what we can expect from the 50th special, we are really fucked

PS: I bet you can tell that I really didn't board the Clara train