The first is that the White House will be held by Ds for three full consecutive terms for the first time since the FDR / Truman regime ended in 1953. National polls may be tight, but they obscure a grip on the electoral college that Ds are unlikely to lose for at least another cycle or two. For this reason, the left, insofar as it relies on Ds for political representation, is about to experience its most extended period of executive rule since the early-to-mid twentieth century.
Second: all of this means that the American left is about to be confronted -- as it has not been in over sixty years -- with the political limitations of the D presidency. The usual contrast of R presidencies will be less relevant and exonerating; it will have been over 12 years since the last one by the time Clinton or Sanders runs for re-election, and within the new norm of D control of the White House a GOP challenge will seem a less urgent and threatening alternative. Meanwhile, we'll have experienced first hand what Ds are willing and able to do given over a decade of executive power.
The only possible outcome I can see here is mounting political pressure among the left to change the status quo of Democratic governance. Control of the White House has dominated Democratic priorities for decades, but will yield diminishing returns for those who are dissatisfied with our current politics. There are ways that this dissatisfaction could be channeled into reform efforts amenable to continued alliance with the Democratic right - such as a shift towards focus on downticket races, or a drive to expand executive power - but it seems just as likely to increase adversity between the two factions, fomenting internecine struggle over control of the party or even its complete abandonment.
I suspect, in any case, that the emerging factionalization of the Democratic party isn't just a fleeting symptom of primary competition. It's a proxy for a much deeper schism that's going to gape wider and wider with every passing day under our next president, in ways that we haven't seen in a long, long time.