The main strike against a review or karma (think likes on Facebook or upvotes on Reddit) is that it is a barrier to entry. New players or GMs will have a difficult time finding a place in Roll20 if they need good reviews or some specific level of karma before others will accept them as players or GMs. Just browse through Looking for Group, and tell me there aren't far more players than GMs. If there is an arbitrary requirement, you will be preventing GMs from running games until they meet the requirement. Which means newbie GMs won't have any leeway to screw up, run a perfect game or no good review or karma for you. When you lower the amount of people willing to try GMing, you'll increase the number of players looking for games. New players are doubly screwed. Not only will they be posting in LFG begging for a GM, they'll be begging for good reviews or karma too. If you doubt that this would be a problem at all, that reviews and karma wouldn't be a barrier, I simply point to the World of Tanks forums and Reddit. People were so abusive to the karma system in World of Tanks that you can no longer downvote a post, you can only upvote it. Much like this forum. Give people a means to be mean, and they will. Reddit is the same, you can up or downvote threads and comments. Brigading is quite common, where people will make sockpuppet accounts (extra accounts) or gather up their friends, and mass upvote or downvote threads and comments. It's a popularity contest, and yes, people can and do go out of their way to get what they want. Does a review system, or a karma system, really work if your choices are either to remain silent or post a glowing review/upvote? How do you know if a review is fair? Or that the upvote/downvote is based on the reviewee's actions and behaviour, and not based on the fact that the reviewer loathed the race the reviewee played that one time? Do you allow a reviewee to have a rebuttal? Or allow the reviewee to upvote/downvote the reviewer's upvote/downvote? How do you deal with people who lie? That brings me to the next point. A simple review system is too simplistic to be useful, except as an arbitrary barrier to filter out people you don't have time to vet. A review system that has context requires human intervention on a regular basis to keep things from getting out of hand. How does that intervention get handled? Are we to pay Roll20 to hire extra employees? Are we to set up a player council to oversee it? Both? It would require quite a bit of work to implement and oversee, more work that it would take people, themselves, to vet one another. It's far too easy, and it will happen, for people to abuse the system.