Politico: 5 Reasons You Should Take Rand Paul Seriously

A pretty good list.  I think #1, #2 and #5 are most important:

He has a stronger organization than any other Republican

Paul starts with a built-in base of libertarians that comprises at least 10 percent of the GOP electorate, and his boosters have made tremendous inroads in state parties around the country.

They may be a minority, but they are a devoted one. Paul supporters will drive farther and work harder than any other 2016 contender's core backers. They also tend to be younger and engaged on social media and the blogosphere in ways that people who support someone of the older generation like, say, Jeb Bush are not.

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He's perceived as principled

Grass-roots conservatives in the early states loathe career politicians as much as ever. There's a real appetite for someone who doesn't always do the politically prudent thing.

The filibuster was a seminal moment not because it changed the conversation on drones but because it showed that Paul cared so deeply about something that he was willing to not urinate for 13 hours. Even liberal critics, from Minnesota Rep. Keith Ellison to Bill Maher, praised him for fighting to support what he believes in.

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He's more cautious than voters realize

Paul often speaks carefully and gives nuanced answers. It's an acknowledgment of sorts that if he wants to be a mainstream leader of the party, he needs to be careful about offending large swaths of Republicans.
His immigration speech is a case in point. An early draft obtained by The Associated Press prompted the wire to report that he would endorse a "path to citizenship," but when Paul delivered his speech, he avoided that term. Afterward, he and his team offered conflicting explanations but stressed he doesn't support "amnesty."

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He appears to have fewer skeletons than his father

Ron Paul faced attacks from the right over racist statements decades earlier in newsletters that bore his name, his criticism of Ronald Reagan in the '80s and suggesting that the CIA under President George H.W. Bush was involved in drug trafficking.

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He can play the inside game in a way his dad never could

After introducing several bills during his first two years in the Senate that went nowhere, Paul has become a more savvy legislator.

Nowhere is this more evident than in the alliance he has formed with Minority Leader Mitch McConnell (R-Ky.), who backed Paul's GOP primary opponent in 2010. Paul's campaign manager that year, Jesse Benton, is now running McConnell's 2014 reelection effort.