Jim Geraghty points out that Alan Keyes was invited to the last Republican debate but was ill-treated, while similar-polling candidates on the Democratic side (e.g. Dennis Kucinich, Mike Gravel) were not invited to the Democratic debate. He doesn't mention that Duncan Hunter and Tom Tancredo were also invited despite not performing well in polls, but that further confirms his point.
His explanation is that they wanted to make Republicans look bad by including the more radical elements but without doing so with the Democrats. While I'm certainly open to such an interpretation (bad motives abound in this world), I'm curious if there are more charitable explanations of why they would have treated the two parties differently on this issue. I can't think of any, but I'm curious to hear any plausible suggestions, because I do think it's a bit strange.