Is there some hidden timing/stamina system for the two-handed axe? Sometimes it doesn't swing when I click, and other times it swings as rapidly as I can click. Is there some timing that I'm missing? Is the grunt the character does supposed to tell me something?

I had the same issue... At the moment Im not playing till the new patch goes up. we have to wait it seems

From what I gather, the grunt was supposed to be an audio clue that the character is ready to attack again. However, you can exploit the fact that using light spell resets the timer and you can attack very rapidly differentiating between attack and magic buttons.