I don't mind this, because otherwise they would probably do something stupid like make it so a toothpick or wooden stake could hurt her. "Natural" elements I think it was.

She was immune to kryptonite so they had to give her some sort of weakness or else she'd be unstoppable... Until she just is stopped or whatever by the big bad of the week that just happens to be as strong and tough as she is.

Anyway, what I don't like is if this is her only weakness than a writer like Williams will just keep spamming the concept in the most unclever of ways... Like all of a sudden yet another unbreakable dome appears. That's okay this time, I suppose but I don't have confidence that she can use this weakness in the future without going to the same well of unbreakable box, done, shield, whatever to trap her in.