The Enthusiasm Gap
Thats my new phrase. Its my description of the delay between being excited about something that goes into an entertaining product, and then seeing it through to the final version.
For example, in Kudos, I decided that if you go to a nightclub, the post mortem screen will briefly play some dance music and have flashing lights. I thought this would be kinda cool, unexpected, and a bit of a laugh.
I still do.
But its tough to keep that same spark of an idea throughout its implementation. That involves:

Finding some suitable dance music sounds, buying them, downloading, converting to ogg, and setting them up.
Recoding the music so it pauses during this new sound.
Looking at some video of nightlcubs for inspiration.
Coding a system of flashing lights.
synching it all up to the nightclub screen and checking it works

By the time you have done all that, its tough to remember why it was so cool. Its similar to a comedian who has a standup routine I suppose. Do they REALLY think any of its still funny after going through it 100 times? It's a big problem, and its worse if you work alone, because there is nobody to ask "is this still cool?"