As I understand it, karma is anonymous like it is to prevent counter-rate downs and the like. This comes, of course, at the cost of accountability, which isn't necessarily bad. Unfortunately, the totally ambiguous nature of ratings compromises some of the rating system's functionality, which is to encourage what the community considers to be "good" posts, and to punish perceived "bad" posts (as well as make them invisible).
What if, when you rated someone, you could also type a short phrase, or select choices from a drop-down menu? Having a menu would be more limiting, but would also require much less storage. The phrase would explain the rating you gave - for rate ups, you could choose things like "entertaining, interesting, well-written, thought-provoking", etc., and for bad ones, "misinformed, poorly written, troll, argumentative", etc. You could include an "other" option for both, too.
Another benefit is that having a list of reasons to rate something up or down could encourage people to rate according to the admins' vision for rating.
The reason would be associated with the rating, and a list of all ratings/reasons on a given post could be viewable either by the author of the rated post, or by everyone - it might useful, for instance, to see that someone was rated down for including incorrect information. This way, rating would retain its anonymity while providing additional helpful information. People who are afraid of karma retribution, or bumping a thread, won't have to add a post to a thread just to explain their rating, but at the same time, the person who was rated would know why, and be either encouraged or discouraged from posting in the same style in the future.