ohmikeghod the Venerable wrote:
The truth is that all DK trees are viable. Go with what feels right for you.
While this is true to a point, there are some things you can take into consideration while choosing.
First, just to restate what ohmikeghod said, all three trees are viable. DK's as a whole level better than most specs/classes by default. However, Blood and Unholy, at least for me, felt like they came into their own a little sooner than Frost. Blood's self-healing capabilities, paired with its damage and early maturity make it "the" leveling spec, IMO, with Unholy closely behind with its pet and AoE damage, followed by Frost.
That having been said, I am Frost now (at 76), I did most of my leveling as Frost simply because I liked it, and I've never had a problem with it.