I think thats an important addition. A trusted reviewers + should be worth more then a first time reviewer. Their worth should increase with becoming more trusted. Works both ways to, if a trusted reviewer gives a - then 4 points taken of.
Also I think it would help if on a reviewers review page it said when they were last logged as well as saying date joined. If they both are the same then at a glance you can make a better judgement call on if it is real or fake if they left a review in 2008 and not been logged in since.