Over at SeaOfStarsRPG a recent post discusses The Problem of Immunity.
Specifically, how really nothing should be truly immune to any particular effect. Even paladins can be scared (they just deal with it better), and even fire elementals will burn if you manage to throw enough fire on them. In real life there really isn’t much that is absolute. Even the best insulators found will, if sufficient electricity is pushed at them and no easier path is available, allow the electricity through. As the picture to the side shows, it can look pretty cool, too.
In fact, some time ago when I was working on Energy Resistance in Echelon and came to much the same conclusion. The difference between ‘practically immune’ and ‘actually immune’ is significant at a philosophical level, but I don’t think it’s really important at a game level. [The numbers used in that post may be revised and horribly out of date, and I do talent design somewhat differently now, but that's an implementational detail that isn't really important here as much as the thought behind it.]
I’m not entirely sure how to approach effects that aren’t straight up damage. As said at SeaOfStarsRPG, just giving a big saving throw bonus is boring. The solution there (reduce penalties from fear effects by the lower of half the paladin’s level (rounded up) or the paladin’s Charisma modifier) looks workable, my first inclination would be to reduce the stage of the fear effect (panicked to frightened, frightened to shaken, shaken to mildly worried) in order to stick with existing effect descriptions.
I hadn’t considered at all what to do about sneak attack, though I suppose it could be considered in much the same manner as energy damage… should it perhaps apply to critical hit damage as well?
Energy drain in my campaign I am probably going to treat as systemic damage (makes you easier to kill for a time, but you eventually get better if you don’t die). In this case it gets very easy, you just have negative energy resistance and you’re done.
Overall, while I think that absolutes may make for easier monster or scenario design, I don’t think they necessarily work very well for world verisimilitude. I don’t mind cranking up resistance to a high level — ‘practical immunity’ — as long as I know it can be beaten if you put enough effort (and fire) into it.