2008 featured a once in a generation candidate, and Hillary Clinton. The race energized the democrat party, plus it happened two years after an unpopular president spurred major midterm victories for democrats. The stage was set for a historic primary.
There is very little excitement or interest in 2016's race because everyone knows who is going to win, and the field was/is incredibly weak.
In short I wouldn't extrapolate low primary turnout to the general election for democrats. It's going to be a huge election. I can't wait until convention week. Assuming Trump is the nominee, imagine the disaster the RNC will be - from the prime time speakers to everything else. Compared to Bill Clinton, President Obama, and Hillary Clinton at the DNC.