My first thought with the HP dice pool system was that no 6's is a catastrophic failure, each 6 adds a benefit (so a sufficiently critical success can even swing the tide of battle), but all 6's are removed from the dice pool, so you *really* don't want to use the dice pool too often, because eventually no matter what you're going to get screwed (unless there's a mechanical way to add dice back into the pool).
My first thought with the HP dice pool system was that no 6's is a catastrophic failure, each 6 adds a benefit (so a sufficiently critical success can even swing the tide of battle), but all 6's are removed from the dice pool, so you *really* don't want to use the dice pool too often, because eventually no matter what you're going to get screwed (unless there's a mechanical way to add dice back into the pool).