All three specs are decent for leveling.

If you go with 5x Blood, you have the ability to heal through just about anything.
Your DPS will be Heartstrke + Obliterate, with Rune Strike as the fall through.
Lots of blood worms, Rune Tap every 30 seconds.

If you go with 5x Frost, you'll primarily Howling Blast + Frost Strike.
Blood Strike and Plague Strike for the other runes, occasional Blood Tap.
Eventually, Necrotic Strike will be rather strong.
Basically Howling Blast and Frost Strike as much as possible.

With 5x Unholy, you'll have permanent Ghoul pets.
Pretty much, get your Frost and Unholy diseases on the target, then spread them.
Forget the names of those abilities, have been Frost/Blood more recently.
The fall through is Death Coil.

All three specs, round-robin Death Grip and pull things in.