From Tateru Nino’s blog:

Sorry, Portal. For nearly a half hour, you asked for wit and ingenuity. Then right near the end you started asking for reflexes. Sorry, I don’t actually have any of those.

Guess I won’t be able to finish the story. A shame.

Switching metaphor or required skillset half-way through a game (or story, or application, or whatever media experience imho) seems like an excellent way to loose audience.