Despite the controversy around him, I would say that Barack Obama has been a clear advantage.
First: Obama has allowed the Democrats to solidly claim that they are the party of diversity. To their credit, Republicans have now run several ethnically diverse candidates and several women. (They brag far too much about this given the clown car that is running for 2016: It's easy to be diverse when you have enough candidates you need at least two hands to count them). This is a huge advantage as the country becomes more Hispanic.
Second: Obama was substantially more progressive than Clinton. While Obama has been very limited in many ways, he did win many major changes like the ACA (whatever its flaws).
Third: Obama has been able to challenge the stereotype of a stuffy liberal. The successful candidates for the Democrats have been cool in some way: Clinton was cool, Gore was not, Kerry was not, Obama was. (As an aside: This year, I really appreciate how Sanders has come off as the cool grandpa. He doesn't quite understand what a fist bump is, but you don't care).
Fourth: Obama's message of hope has galvanized a generation of more and more progressive youth voters. Millennials are largely rejecting the Republican Party.
Obama's election may be viewed as a watershed moment in the history of the Democratic Party within thirty years.