Lawrence Lessig is a serious Presidential candidate, polling when his name is included in polls above candidates including Jim Webb and Lincoln Chaffee who will be invited to the Democratic debates, and who has raised over a million dollars in only a month after declaring his candidacy.
His reason for exclusion from the debates is that even after announcing his candidacy, Lessig has not been formally 'welcomed' into the race by the DNC, and as such has not been included in some polls which are used to determine eligibility for the debates.
Although I am myself not a supporter of Lessig, I believe he is a serious candidate and party committees should not be in the business of picking winners before the debate.