Creating comments and nodes with Services 3.x and JSON
The following JSON appears to be the minimum you need to post a comment:

{
  "nid":"1",
  "subject":"Comment Subject Text",
  "comment_body":{
    "und":[{
      "value":"Comment body text"
    }]
  }
}


Since there is no user, commenting on this "example.com/node/1" needs to be turned on and permissions for anonymous users to post comments need to be set.