While I do agree that karma (or whatever the final term will be) shouldn't be obvious to the casual observer unless they have some sort of magical means to see into the soul, I don't think the basic idea is necessarily problematic. It's just about how you do it.
The problem with not having karma, or something like it, is that there is no penalty for stealing Source, which would clearly give "evil" (I'm simplifying here) characters an unfair advantage. Unless they scrap the stealing Source mechanic, something has to be put in place to balance it.
I posted
an idea of how to do it over at the uservoice game ideas site, and a guy called Andrew Ramsey filled in (a lot of) different takes on the same principle. I think it's a sensible way to balance Source theft while avoiding the pitfalls of an obvious "hollow-eyed and shrivelled evil monster" route.
