Once a powerful spell in BC, it seems to me like the drain life spell has been long forgotten ever since WotLK. I don't know if the coefficient got nerfed or what (soul siphon did get nerfed) but this spell just isn't what it used to be.
It's a shame, too. I really think it's time Blizzard came back to this spell and gave a hard look at it. Changed properly I think an improved drain life could mean great things for both PvE and PvP.
In PvE, drain life would be a great substitute for shadowbolt in the affliction rotation. Much like how Blizzard got death strike in the blood rotation instead of constant obliterates, drain life should be beefed up to be worth using in the affliction rotation instead of shadow bolt.
Shadowbolt is a destruction spell. It makes no sense what-so-ever for the largest portion of damage coming from any one spell in the affliction rotation to be a destruction spell. Why do all three warlock specs have to do most of their damage from destruction bolt spam?
Give us different playstyles. Drain life in the affliction rotation would make it more unique from the demo and destro rotation instead of the same old bolt spell spam.
I think what will bother a lot of people about this suggestion is the fact that drain life also heals and therefore "isn't worthy of doing as much damage as pure damage spells".
However, this isn't true. For one, death strike also heals for a lot and it was worked into the blood rotation anyway. Second, drain life doesn't have to do as much damage straight up as shadowbolt. Give affliction talents that gives dots a bonus for using drain life and things of that nature.
In PvP drain life could improve the warlock situation in many ways. After siphon life and immolate were removed from affliction, affliction is suffering in pvp because after dots are applied there's no filler damage to do except shadowbolt/searing pain which just isn't feasible.
More importantly drain life as PvP spell makes warlocks, warlocks. I know one of the criticisms of buffing drain life in pvp is that warlocks will just be "drain tanks" and "warlocks are not supposed to be tanks, they're supposed to be kiters now".
Well, let's be honest here. Has Blizzard's attempt to make warlocks kiters instead of tanks had any success in PvP? No, it has been a massive failure. The only way warlocks are going to be adequete kiters is if you copy/paste the frost tree and if you ask me, the mage and warlock classes have already been homogenized enough.
Look at the only new spells warlocks got in WoTLK: shadowflame and demonic circle: teleport. IE watered down cone of cold and watered down blink.
Warlocks being "caster tanks" in pvp was one of the few things that made them unique from mages. Mages have always been known in PvP for having strong snares, are highly mobile and set up huge burst. Warlocks, on the other hand, mostly used dots for damage, and used fear and their beefy tanking skills for defense.
But instead of making warlocks and mages unique in PvP, Blizzard has tried to make warlocks just like mages. Fear is no longer a reliable CC after the huge 3.1 nerf. Affliction now only has 1 more dot than a fire mage after the equally huge 3.1 nerfs siphon life and immolate. Warlocks are "no longer supposed to be tanks in pvp" but rather "kiters".
Let warlocks do what they've always done best: Dot, fear, and drain tank. Blizzard's attempt to make warlocks pvp like mages has just been a massive failure and warlock's arena representation shows it.