One major problem I never hear anyone talk about is that each of the people who want to vote third-party want to do so for very different reasons, often at political extremes. Which is why there's no "third party" but a bunch of very different tiny parties with very different goals and no electoral pull. So even if you convince every undecided/independent/major party hater voter in the country to vote third party, their votes will never coelesce under any one party enough to make any difference. Of course it all leads back to winner-take-all and first-past-the-post, which are the real enemies of truly representative democracy.