I would agree about the people voting on the Worldcon, but that tells me that the attendees may not understand completely that no matter which site wins, voting gives you a supporting membership and if your site wins you pay the same to upgrade anyway (unless you pre-supported which if you bother to do that you are probably a regular attender).

Maybe a campaign to explain that better at the convention would help. Also has there been any thought to eliminating the fee to vote and allow all paid members to vote? I know that limits the upfront cash for the winning site but possibly there are other ways to make that up.