Great plugin! You should be commended for all the time and dedication you have poured into it. Your work clearly stands out above the crowd.
I noticed an issue when manually removing a user's achievement. Upon removal, the karma value does not drop. Then if you re-award the achievement, the karma value is not added. This leads me to the notion that karma values are a one-time addition and there is no "checking" to see if karma should be added or subtracted depending on if an achievement is removed or re-awarded to someone. Is this a glitch, or perhaps by design? Perhaps there's a reason it works like this that I'm not aware of.
Keep up the great work!