Voted down just to ruin your expectation of positive karma (not really).
SHA-1 of my karma prediction for this post: 4154a31be35736ebed12ba65cbcb89cabc08b375
ggreer@carbon:~$ echo Positive but less than 10 | shasum 4154a31be35736ebed12ba65cbcb89cabc08b375 -
0 isn't positive so I was wrong.
A fun game you can play on LessWrong is to stop just as you are about to click "comment" and make a prediction for how much karma your comment will receive within the next week. This will provide some quick feedback about how well your karma predictors are working. This exercise will let you know if something is broken. A simpler version is to pick from these three distinct outcomes: Positive karma, 0 karma, negative karma.
What other predictors are this easy to test? Likely candidates match one or more of the following criteria: