While a dislike button, and collapsing posts with too many dislikes can be useful to keep topics relevant, the risk of abuse is high, unless you limit it to users with a strong commitment to this forum*. In effect, you would be adding a sizeable group of 'moderators-lite', which may not be a path this community is ready for.
* If its easy to get, trolls will register multiple sockpuppets to let them game the system.