Length - The word count in a review had a definitive impact on how relevant it was considered. There was a clear correlation between the number of words contained in a review with its position in the Most Relevant rankings.

Keywords - Using the business's name in the review led to an increase in the post's relevancy ranking.

Specific - A unique story or example based review was more likely to rank higher than reviews with generic content. In the example below, this rather long but specific review was listed first by Google.

Local - A review by a user who has a history of reviews in the specific local area is deemed more relevant than a user with either no reviews or one with reviews from different geographic locations.

Time - Reviews seem to lose relevancy over time. In this example, the second review shown has been labeled NEW as it was posted 2 weeks ago.
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Votes/Likes - Unless a review gets 10-20 votes/shares on the post, it won't help much to get a few. (Unless a review is viral-worthy, it won't get many votes or shares anyways)

Response from owner - Owner responses did not seem to affect placements on the Most Relevant list.

Negative - Contrary to popular belief, Google does not rank negative reviews higher than positive ones. In all of the tests, positive reviews meeting the criteria outlined above are just as likely to be considered relevant as their negative review counterparts.