I think the new system is fair. Costing a green in order to give a negative is an effective deterent against lower standing members giving bad rep for fun or frivolous reasons. As for those who have 5+ green blocks, I'd say that they're rep would not be where it is if they were accustomed to that type of poor judgement.
I had someone who wanted to give red to the poster above me but accidently redded me, it was an accident and it happens. you cant get every rule made for you.