Allowing Clinton to label herself a "progressive" (because she believed in progress) and conceding that Obama was also a progressive, Bernie all but conceded the nomination. By failing to distinguish himself and define his cause as something other than what Obama and Clinton have offered, Bernie undercut the raison d'etre of his vaunted "political revolution."
As explained in Chipster's Journal, the word "progressive" has a defined meaning in U.S. political history. It includes nice and decent things like raising standards of living, conservation, and so on. More importantly, however, progressivism was and remains a rejection of Jefferson individualism and its political correlative of State Sovereignty. It stands for a concept of government as the custodian of common resources and socio-economic welfare with powers to regulate the use as well as the distribution of wealth.
If Bernie's "democratic socialism" did not mean this it meant nothing. Clinton and Obama are not progressives but globalist neo-liberals (to say nothing of warmongering). Bernie was not obligated to delve into the arcana of political philosophy -- arcana that are beyond the mental capacities of most U.S. denizens in any event. But it was incumbent on him to draw a line between himself and his opponents and to assert, at the very least, that while Obama and Clinton might like to think of themselves as "progressives" they have both utterly failed at enacting anything on par with either Roosevelt.