Democrats are making breakthroughs on issues of race and gender: Democrats have overcome long-standing US racism and sexism to get behind a black candidate and female candidate in enormous numbers. They have faced some difficult issues about gender and race - questions such as: Can a woman be respected as Commander in Chief? and Is a member of a black community too much a part of an interest group to represent the entire population? and they have answered them more than adequately.
In addition, we could consider the lengthy battle not as a negative thing, but as:
The long nomination process is a PR coup for the Democratic party: Democrats are energized like never before. They are voting in record numbers. The numbers voting for Democratic candidates is much higher than for Republican candidates. This means that more Democrats are registered and are on lists. Come election day, Democrats have huge lists of people to call in their "get out the vote" push. This year's primary process has been extremely beneficial for our chances in November.