I think for me it was the direct comparison that broke it to me, because they aired so close together.
PoI and SPN basically did the same thing, just that one show was breaking in, the other was breaking out. Both group of heroes had a whole army against them, and still SPN was written like freaking Augsburger Puppenkiste, while PoI was written like a multimillion dollar movie.

The thing is, SPN was never good at those scenes, but they saved themselves through their character interaction, at least somewhat. But if that turns meaningless, too, there's nothing left.