One complexity I struggle with is developing potential. Say 'B' is really talented, but also spends a ton of time doing seemingly-unrelated stuff. Maybe chatting with friends on IRC, reading fantasy and gaming blogs, watching YouTube, etc. is part of how the they end up getting work done. But I also think about how maybe just a little less of that could result in them becoming a top performer, because they have the talent and could apply it a little more. Though I completely get that people need "down" time in this business, I've also seen people become exceptional with a little coaching about planning/focusing and time management. The topic of how much time is spent on "other" stuff is often very difficult to discuss in a way that doesn't come off as reprimanding.