Well I first leveled Darsithis years ago when all 3 trees tanked/dps'd.
But when I leveled in Cata and MoP at each expansion I went frost. Despite everyone saying "I can round up a billion mobs and AoE them down as blood" I found I'd kill all those mobs by the time they managed to gather them up.
Frost 2H ftw. Take glyph of dark succor so you can heal yourself every time you kill a mob if you find you're taking a lot of damage.
I really loved leveling with DK an do quests. One of the best part of my WoW adventures at wotlk quest with frost spec. Ahh good old days. When first tried the dead siphon, i just watched the numbers, it was awesome. Wish that it was as useful at lvl 90 as it was when i leveled.