A problem is that karma attempts to capture orthogonal values in a single number. Even though you can reduce those values to a single number they still need to be captured as separate values e.g. slashdot karma system for a half-assed example.

Karma seems to roughly fall into one of three buckets. The first is entertainment value e.g. a particularly witty comment that nonetheless does not add material value to the discussion. The second is informational value e.g. posting a link to particularly relevant literature of which many people are unaware. The third is argumentative value e.g. a well-reasoned and interesting perspective. All of these are captured as "karma" to some extent or another.

Objections are that this makes it difficult to filter content based on karma, which raises questions about its value. If, for example, I am primarily interested in reading hilarious witticism and interesting layman opinions, there is no way to filter out comments that contain dry references to academic literature. Alternatively, if I lack an appropriate sense of humor I might find the karma attributed to immaterial witticism inexplicable.

Even if a clever system was devised and ease of use was ignored, there are still issues of gaming and perverse incentives (e.g. Gibbard-Satterthwaite theorem et al). To misappropriate an old saying, "karma is a bitch".