Because Drain Life deals damage and heals, it receives a 50% penalty from +damage gear. The total bonus is then distributed amongst the five ticks of the spell. For example - if you have +100 spell damage, each tick of Drain Life will transfer +10 HP from the target ((100*0.5) / 5).
Drain Life combined with [Life Tap] can allow a Warlock to act as a tank.
Patch 2.3.0 (13-Nov-2007): The amount healed is reduced when the warlock is affected by healing reducing effects (e.g. Mortal Strike, Wounding Poison).