If there is ever an interest in implementing something like this in the future, I used to belong to another SMF forum that had this enabled. At one point they did have it set so you could only give karma points, and not take any away. I don't know if it was a setting or if they just figured out a way to hide it, but I do believe it's possible.
Also, they ended up implementing a required comment field when giving or taking away karma, so you had to provide a reason. That really helped to keep people from "disliking" posts for no good reason.
I'm friends with the admin of that board, and would be happy to ask him how these things were implemented.
I'm neutral on the whole thing.