It's not worth it, but I am waiting for Talon to come here and tell us we're all n00bs. However, he does have some good advice on how to juggle your mana for Destruction. Even more, at 30, you didn't have Dark Pact, so the same mechanism of Life Tap->Drain Life and Drain Mana will still work for you.
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improved shadowbolt is better i guess even if i dont know fully what it means.
When your Shadow Bolt crit hits the target you cast it on, a debuff is applied to will increase the amount of damage done by Shadow spells. The four non-periodic refer to DD hits. If you hit crit an enemy with the shadow bolt, and then cast 4 DoT on them, you can cast a 5th and the debuff will remain. If, however, you hit them with the debuff, then 4 more bolts, the debuff will go away.