All the way back to last summer there was talk about how Sanders fans were so much more enthusiastic. Now that we are well into the primaries, this has gone from an observation to a fact.

So far in the Democratic primary, there's only one set of circumstances in which the party is producing strong voter turnout: When Bernie Sanders wins a state, tons of voters are showing up.
In Colorado, Kansas and Maine, Democrats have actually surpassed the turnout of the 2008 primary contests, while the caucuses in Minnesota and Nebraska only narrowly missed. Sanders won all five states.
...
In every state that Hillary Clinton has won -- which is most states -- the party is seeing a significant decline in voter turnout from the 2008 election. Half as many Texas Democrats came out to vote this year than in 2008. In South Carolina, turnout was down by a third. Virginia? Down by one-fifth.

This trend can be seen on a micro-level in Michigan, where Bernie engineered his most dramatic upset.

Consider the voter turnout shattered records when more than 2.5 million people cast ballots. Sanders won big just about everywhere, except in Wayne, Oakland and Macomb counties. And while a big showing in Wayne County and its city of Detroit often spells victories for many candidates, the turnout was 25% in Detroit and 31% in Wayne County, while the statewide total was closer to 40%.

It must be acknowledged that heavy turnout in primaries does not mean heavy turnout for the general election.
Nevertheless, primary turn out is still a good measurement of enthusiasm.

But it's still a meaningful way to gauge the excitement among a party's base. Ask anyone at the Democratic National Committee whether they'd like to see good turnout numbers in the primary, and they'll tell you that, yes, of course they would.

As for electability in the general election, it's time to look at demographics in a more realistic sense. The NY Times hints at it.

And despite the seemingly inexorable demographic rise of Hispanic voters, the American electorate is still overwhelmingly white. Some analysts said they believed Mr. Trump could even exceed Mitt Romney’s 59 percent share of the white vote — winning over disaffected Republicans and even working-class Democratic men, and putting Democratic-leaning swing states like Michigan, and potentially Pennsylvania, in play...

The Democrats can't afford to lose Michigan and Pennsylvania. Not when the South is a solid Republican lock. Ohio has a smaller percentage of black voters than Michigan.
Someone needs to be able to sell to working-class whites in swing state.

The New Hampshire primary provided some encouragement for Democrats. Among white voters without a college degree, Mr. Sanders won more votes than Mr. Trump....In another contest, the Massachusetts primary on Super Tuesday, Mr. Sanders won 36 percent of that group’s votes, in contrast to 27 percent for Mr. Trump. Not far behind, Mrs. Clinton received 24 percent of that vote.

Democrats are going to need the base to turn out because independent voters are a hopeless cause for the Clinton campaign. They flat out do not like her.

Between March 2015 and December, Clinton's net favorability -- those viewing her favorably minus those viewing her unfavorably -- sank from plus-3 among all voters to negative-8, an 11-point change. But among independents, that figure went from +4 to -27, a swing of 31 points.

As an example, Sanders won Michigan almost exclusively because of the independent vote.

YouGov expected Sanders to beat Clinton by 38 percentage points among independent voters participating in the open Democratic primary. He won those voters by 43 percentage points. But no one expected independents to make up 27 percent of voters; YouGov expected about 12 percent.

All general elections are "open primaries".