At the close of official 4e development, The Rules Compendium (p225) had the following to say about damage, condition and effect immunity:
“Some creatures are immune to certain effects. If a creature is immune to a damage type (such as cold or fire), it doesn’t take that type of damage. If a creature is immune to charm, fear, illusion, or poison, it is unaffected by the non-damaging effects of a power that has that keyword. A creature that is immune to a condition or another effect (such as the dazed condition or forced movement) is unaffected by the stated effect.
Immunity to one part of a power does not make a creature immune to other parts of the power. For example, when a creature that is immune to thunder is hit by a power that both deals thunder damage and pushes the target, the creature takes no damage, but the power can still push it.
This is basically fine. It’s in line with the general 4e design vibe, where players will often find their efficacy reduced, but almost never completely nullified. But as many people have noted, one of these things is not like the other: poison.
Unlike charm, fear and illusion, poison is also a damage type, so it gets hit by both clauses; a poison power has no damaging or non-damaging effects on a poison-immune creature.
My guess is that this was mostly unintentional. I doubt any designer sat down and decided it made sense for poison immunity to make you immune to the “hammer” part of a poison-laced hammer to the face. Rather, I imagine this is aimed at mundane/alchemical poisons. After all, it would also be a bit of a fumble if poison immunity simply did nothing to protect you from, say, a poison that inflicts weakness but deals no damage.
Problem is, of course, you can also get the poison keyword by adding any amount of poison damage to a power. Since you can apply poison to weapons, it’s about the easiest addon of this sort in the entire game. Does this make it powerful enough to justify the discrepancy? Maybe at very low levels, but the cost of consumables at that level is easily enough to balance it out. (And immunity is unlikely to show up anyway.)
So, while not all damage types are equally powerful in 4e, I’m not okay with poison being this much of a low outlier. This game thrives on balance, which is super important since your power level often directly represents your ability to affect the story. Poison’s power and feat support aren’t good enough to justify such a huge weakness, making it essentially a trap for players who don’t know any better (or who think a poison-user is a cool concept). And I hate build traps!
What can we do about it?
It would have been a good idea to use separate keywords for poison damage (toxic, maybe?) and being a poison. If you want to take the time, you could definitely revise existing monster stats and player options to reflect this. That would be the most thorough and clean solution.
But… I’m don’t really want to re-publish a few hundred tweaked monsters and a whole buttload of powers, and I’m assuming you don’t either. So let’s assume that airship has sailed. Under that condition, here’s my rules patch to rectify this imbalance in as simple a way as possible:
Creatures which are immune to poison damage are not immune to the non-damaging effects of powers with the poison keyword. However, they are immune to all effects of poisons described as items (for example, alchemical substances).
When poison is applied to a weapon or projectile, a creature which is immune to poison damage is immune to all effects conferred by that poison. Effects which are not conferred by the poison (such as base weapon damage and non-damaging hit/effect properties of a power used to deliver the poison) are not affected by the immunity.
These changes are designed to make poison a more viable damage type, since full immunity to damage is normally very difficult to acquire.
Is it especially elegant? Goodness no. But it does keep poison specialisation from becoming a liability after Heroic tier, which is absolutely my goal.