Project size and churn rates for completion are good for preventing slacking, but interest in your work is just as important. Having a high churn rate doesn’t mean you’re doing well;

You could have weeks and weeks of never working on your saturday morning project, and if you got everything you hate done you’d still have good churn.

Scott and Cal, I find it interesting that this system is similar to a feature of Pivotal Tracker (a project management software) that automatically calculates how effective each individual contributor is being.

They use a easy, medium and hard rating scale of 1, 2 or 3 points respectively and it works well — this could very well be a good answer to the problem of rewarding harder/larger projects.

Also has the benefit of helping you tackle the harder stuff first (feels more satisfying to earn 3 points instead of 1).