I have always thought a poll needed a minimum of 20 votes, for or against, for it to be valid. I don't remember if that rule is only spelled out on the Beta forum or was something told to us here by a previous CM.
Edit: yep., these are the full rules from Beta:
* After this, the Moderator will open a poll in this thread. He/She will also make sure that in the opening post, there is a short description of the Idea (as well as in the Thread Title) and all detailed information is included in the rest of the in-depth description in that opening post.
* The poll will run for 14 days.
* During the poll, the discussion can still continue if anyone feels they have something valuable to add. However: No alterations can be made anymore to the idea as it's stated in the opening post, so please refrain from coming up with new ideas at this stage.
* At least a minimum of 20 individual players will have to vote (in favor/not in favor) for an idea to get approved.
* At least 80% of these players will have to vote in favor of the idea in its current state (to avoid forwarding ideas that aren't supported by a good majority of the community).