Every class skill line for DK's are useful to get to level 50. Even if you don't use the skills there, the passives are nice to have.
I am not sure how new you are to the game, but since you wrote 2h magic build, I have to mention one thing. Remember that the weapon skills like 2h, scales with weapon damage, weapon crit and your maximum stamina, while most class skills scales with spell damage, spell crit and max magicka.
As a DK with 2h, you can use noxious breath, venomous claw and flames of oblivion. Every ultimate in the game, scales with your highest stat, regardless magicka or stamina, allthough the flame damage from standard of might wont benefit from physical penetration.

Unlock all of your skills lines unlock each and every ultimate with your weapon skills and do it early nothing is more annoying than going back when your past level 50 and then decide you should have leveled up a particular skill, weapon or Armour ability or passive. At present my DK is magika although I got two handed and one hand and shield to 50 some situations may necessitate using skills outside of what you are normally running. In short experiment with different things as you progress in levels and most importantly enjoy the game after all if you are not having fun then what is the point.