I figure three stars or more is probably fine, if you have less than 3 stars, you are probably a bad person.
Well, yeah - 3 is 'average', since it's the midpoint on a five-star scale.
What would be more sensible than a directly voted rating is one derived from 'liked' posts; but that would have to be figured individually, as a proportion of that person's posts (rather than a simple threshold value - n likes per star) since some of us post way more than others.
No, not really. Karma, at least as Reddit has it, is just a tally of your upvotes - and naturally anyone who posts a shitload will get more, just as a function of posting more in total. I'm suggesting an indication of proportion of your posts which are actually worthwhile, as opposed to just noise.