The most common one is that people like me claim there's no difference between Donald Trump and Hillary Clinton.
I don't know about other Green voters in particular or left-liberals in general, but as for me, that's simply not true. Period and end of story.
Now, I DO say that there's less difference between Clinton and Trump than between Clinton and Stein.
In 2012, I said the same about Barack Obama and Mitt Romney than Obama and Stein, or in 2008 between Obama and John McCain vs. Obama and Cynthia McKinney, or 2004 and John Kerry and George W. Bush vs Kerry and c.
I stand by that, and expect to stand by that in the future. (I won't hold my breath over who Dems nominate in 2020 if Clinton loses, nor will I hold my breath on Clinton being anything other than a generic centrist if she wins.)
You'll notice I didn't go back to 2000. That's because I've voted Green in every presidential election this century, and 2000 is technically in the previous century. I'll have more on Ralph Nader, the man vs. the myths, in the future.
Unfortunately, along with the general "two sides only" of the duopoly appears to come a lot of black-and-white, or better, Manichean, thinking in general.
Once again, in the face of such thinking, I quote Idries Shah:
To 'see both sides' of a problem is the surest way to prevent its complete solution. Because there are always more than two sides.