Other nations could be gallery countries that watch, but are not allowed to vote.
This would create incentives for countries to become certified democracies.
What we should have a is a United Democracies where votes are counted on the formula of "population size" * "human rights" where human rights is on a scale of 1 to 10, 1 being just barely free enough to participate in the UD and 10 being almost perfectly free. The "human rights" category should be only the most basic and most important rights, like those found in the Constitution (except maybe the 2nd Amendment, I don't think we would penalize other countries for deciding differently on that matter). This way, we wouldn't have to suck up to tiny countries like Guinea for no good reason, and we wouldn't have to suck up to dictatorships like China. We wouldn't have to worry about France because they would be too sophisticated to join a coalition if their good friend Mugabe wasn't invited. Countries not meeting human rights standards would automatically have zero votes, because of their human rights score.
We would have to divide the number of votes by some huge number like 10M or 100M or America could conceivably have 3 billion votes in the UD (which would take a hell of a long time to hand count if Gore got snippy).
This would create a direct incentive for countries to democractize and grant basic rights to the citizens, even if they are relatively free countries (like the UK, for example, which would lose some votes over free speech issues). It would not have any affect on dictators like Mugabe, because he knows he's not even going to make the floor value to get into the UD, so there's no point in his trying to democractize. But I really can't see any other international system that would encourage him to make his country freer short of threatening to do to him what Bush is doing to Hussein right now.