2. Why Was He Tapped? Castro is a useful pick for Democrats for several reasons. He helps the party with its appeal to Latino voters, a key voting bloc this year, and respond to the sea of diverse faces the GOP showcased in Tampa. His selection offers a chance to introduce a young man the party sees as having great potential to a national audience -- much like Democrats did with a certain state senator from Illinois in 2004. The other side of that coin is that he's unlikely to overshadow the man at the top of the ticket, as some worried Governor Chris Christie might in his equivalent keynote in Tampa last week. It's also a pragmatic selection. Behind Obama, the Democratic bench is weak. One of the few other party rising stars, Massachusetts Senate candidate Elizabeth Warren, will speak Wednesday night. New York Governor Andrew Cuomo, an oft-mentioned 2016 hopeful, has opted to lay low at the DNC. With few Marco Rubios and Chris Christies available, Democrats are leaning heavily on mayors, including Castro, Charlotte Mayor Anthony Foxx, and Los Angeles Mayor Antonio Villaraigosa, the convention's chair.

3. Is He the Next Barack Obama? Like the search for "the new Bob Dylan," the frequent invocation of the president's name to describe up-and-coming non-white-male politicians -- from Marco Rubio to Artur Davis to Hakeem Jeffries to, uh, Sarah Palin -- can take on a pathetic, desperate, and reductive tone. But there are a few undeniable parallels between the two men: Both were raised by single mothers, attended Harvard Law School, and have sought to transcend racial politics while maintaining a connection to (and base in) their ethnicities. (Castro, interestingly, does not speak Spanish fluently -- though you wouldn't know it from the accent he likes to use when pronouncing names or words with Spanish origins.) And both reach for broadly unifying themes but are happy to throw a rhetorical punch at Republican adversaries. Obama has taken Castro under his wing -- he's been invited to White House summits, sat with first lady Michelle Obama at the 2012 State of the Union, and has now been tapped for the DNC keynote.

4. What Is His Future? Like all skilled politicians, Castro is careful not to telegraph his ambition too clearly. When Time named him a member of its "40 under 40" list and asked where he saw himself in five years, he replied that he'd be mayor of San Antonio. But Castro has been an ambitious local star since high school -- the type of local boy who might come home and make good, but is expected to make good on a larger stage, too. Former George W. Bush adviser Mark McKinnon told The New York Times Magazine, "Julián Castro has a very good chance of becoming the first Hispanic president of the United States." What would Castro's next step be? He faces a challenging landscape in Texas, where Republicans have a lock on statewide offices, but he has loudly criticized Governor Rick Perry and may have an eye on the governor's mansion anyway. Whatever he decides, the DNC slot should raise his profile and give him a boost.