The 'karma' systems are usually limited in that there could be multiple people that helped in a thread, but you may only be able to give marks to one person in the thread, or hand out a certain amount of karma at a time, etc.. (most karma systems have some kind of limitation like this in place, to prevent abuse). And on that note, we found that, just like virtually every other site out there, people tend to abuse the karma system. It's not so bad on social oriented sites, but on support sites where it's intended to be used as a measure of helpfulness...well, you just can't filter out the marks that were given for social reasons. This makes the karma system not dependable for accuracy of helpfulness/knowledge.
With hand-picking promotions, we don't have to worry about the b.s. involved with people giving out positive or negative marks to other people for arbitrary reasons. We allow for nominations and then we allow for thoughts and comments and voting on the nominee, even by his/her would be peers. We acknowledge that hand picking people is a slower process, but overall, it is more accurate.
