While still very competitive, the Democratic Primary is much more predictable than its Republican counterpart. This is a fairly simple result of having fewer candidates: with only Hillary Clinton and Bernie Sanders remaining as contenders, there is only two possible outcomes to each state's contest.
Through the summer of 2015, Sanders, who was serving as an Independent senator from Vermont, was widely seen as a protest candidate, there merely to remind the world that there was a progressive wing of the Democratic Party that might object to the centricism of overwhelming favorite Hillary Clinton. At some point, over the summer and autumn, while the race was mostly fixed on the boisterous behavior on the Republican side, Sanders slowly gained a following. At first, it was mostly the young and the active that were interested, but with ripples coming out from social media, Sanders slowly became a more viable candidate. Without a single event marking the shift, he overtook Clinton in New Hampshire polling, and gradually opened up a wide lead. On election day, the polling was vindicated, with Sanders winning over Clinton with a 60-38% finish. This was an overwhelming victory, especially coming on the heels of Sanders almost winning in Iowa
Of course, there are some caveats with that: Sanders is a Senator in a neighboring state, and many analysts believe that New Hampshire's demographics lend themselves to a Sanders victory. He might have trouble doing as well in a string of states that vote soon, which are mostly located in the south and midwest.
Be that as it may, the results in New Hampshire, along with the Iowa decision, have marked Sanders down not just as a protest candidate, not just as a single issue candidate, and not just as a fringe interest, but as someone who is a serious contender, and who represents a significant and popular political position with the Democratic Party. So, much as with what I said after the Iowa contests: this is going to get interesting.