We go through this to some extent every presidential election cycle, but I expect that Stein and Johnson supporters are going to be pissed that neither qualified to participate in the September 26 debate. Here's why I don't share in the outrage:
1. There are a lot of people who run for President each year. Libertarians and Greens might complaint that their candidates don't get much attention, but they have gotten a hell of a lot more attention than the official nominees for the Constitution Party, the Socialism and Liberation Party, the Reform Party, the Workers World Party, America's Party, the Socialist Party USA, the Prohibition Party, the Veteran's Party, the American Party (South), the American Solidarity Party, the Nutrition Party, the Socialist Equality Party, and independents like Evan McMullen, not to mention a shitload of other people who think they are running for president but have not actually gotten on any significant number of state ballots. Unless you're willing to let everyone who says they are running for President onto the debate stage, you need some criteria for deciding who gets on the stage and who doesn't.
2, The criteria to qualify for the debate stage set by the Commission on Presidential Debates is not on its face unfair. To qualify the candidate needs to be on the ballot in enough states to get a majority of electoral votes and to be polling at least 15% in an average of five polls by major media organizations. Maybe others will disagree and think they should be different (I could buy a 10% polling threshold, or argue over which polls should count in the average), but now is not the time to make that argument. The rules were set months ago. If they were inherently unfair they would have been unfair before you found out that your preferred candidate didn't make the cut.