Two issues that I have
1) It makes Tainted and Finite Life a really nasty combo. Perhaps too nasty
2) I think it is actually simpler if Tainted had no interaction with Finite Life and that the removal of the condition was not affected by Finite Life.
In regards to it then not being as effective against creatures with Finite Life, that would, at least to me, be consistent with what the condition is meant to be. Its difficult to taint something that can't be effected by healing.
I'll have to defer to you on the power level issue; I haven't played as Damage over Time Warlock or played against Necro at all, so I have no real way to judge. You're right that it's a potential combo, but it's not that good with the two tainted cards I've seen so far.
Drain Soul is terrible with Deathlock, because you don't get any of the life gain. The combo with the Spider is okay? The spider doesn't seem like it fits any current Nature book archatypes that would want to give up healing, but maybe a Dark book that currently plays Basilisk might want it.
The situation I'm imagining is a Necro with a creature that taints and does damage, and also a Deathlock. Against an opposing Wizard, neither the taints or the damage are likely to be healable. If the wizard packs a wand of healing, though, the damage won't be healable, and the taints ... might be? Priestess, of course, is a special case.
My suspicion is that "compatable" Deathlock would make Tainted as good as Deathlock makes damage. If Tainted is priced more cheaply than damage, I can see how that might be a problem.
To put it another way, Deathlock already makes normal damage better than vanilla Tainted (mostly. Tainted ignores armor, but is a poison effect.). If it Deathlock also brings Tainted "up to par," is that too good?
Of course, it's possible that Deathlock is the problem card, not the tainted mechanic, but we've never had the tools to make it matter, before. I mention this because it seems like Deathlock makes Bleed as good as Deathlock makes Rot, and that's a clear power upgrade that's more likely to be a balance problem. (Not saying it is, just that it seems more powerful.)
As for your second point... man, I don't know. I would expect that a card that prevents me from gaining life would, in all cases, prevent my life from going up. Maybe I just have that expectation from playing too much MtG?
