RE: rating required to make $$
Generally, you can earn money on reviews that have at least a "Helpful" rating. In other words, reviews rated "Somewhat Helpful" and "Not Helpful" are not eligible to earn income share (IS). (Income share, abbreviated IS, is how we refer to earnings on Epinions.)
That said, I think that even Somewhat Helpful reviews can earn IS in certain categories. Not sure about that.
In theory, the better your ratings are, the more likely it is that people will find your reviews. All things being equal, a review rated VH will get better placement on the site than a review rated H. There was a time when reviews rated SH were not shown to the public, but I think that is no longer true for some categories. Still, VH reviews get much better placement than SH reviews, so it's always best to work toward VH ratings.
I don't think I've ever seen a review with an overall rating of "Most Helpful." It's an accomplishment to get an MH rating from just one member, let alone the several that would be required to give a review an overall MH rating. I don't even know if an overall MH rating is possible.
Ratings do not have to be unanimous. For a review's overall rating, all the ratings are blended together using a complex formula that takes into account the "rating strength" of the people who rated. Not everyone has the same rating strength, and Advisors and Category Leads are given extra rating strength in their categories. It really is pretty complicated.
Good question... thanks for asking!