My problems with this and several have touched on it:

1. This utterly punishes people who usually group with friends. You will receive almost no votes if you routinely group with friends and no matter how good you are if some random players sees you have very few if any upvotes they will think you are a griefer and leave the instance.

2. This does not do anything to address the major issues of some players who intentionally screw up instances to get people killed or go AFK in an instance. There must be some kind of feature to /kick people out AND to invite someone into an instance.

3. There should be a downvote. The way to stop griefers is to make public how someone votes. If all a griefer does is downvote others on purpose it would show and other players would be warned to avoid this player.