Say what you will, but I consider the black veins all over my face and body to be a pretty big downside.
Except it isn't as no one else in the game seem to notice and absolutely nothing of the plot changes.
I notice.
Your argument seems to be coming from a strange place, at least to my reckoning. Do you not care what your character looks like? Do you think other players don't care what their character looks like, in general? Would it suddenly be a bigger drawback to you if NPCs said something like, "Egads, that face!" when you walked by them?
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I'm legitimately trying to understand.
I get the feeling that people want, either:
1. Something good to happen to those who don't use parasites... but they have no idea what good thing should happen or how to explain it in game, or
2. Something bad to happen to characters who do use the parasites... but it should be worse than destroying the look of their character. In which case it should be... thus far, I've only heard a suggestion that it should become increasingly hard not to use the tadpole power in dialogue. Which I guess means a loss of control over personality?
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Don't get me wrong. I certainly think there's room for improvement on the system. I'm just not hearing anything that feels better yet.