I kind of have this silent rule that I'm biased to healers if I'm not healing. Healing is a very dependable role, and it'd be very difficult to clear a lot of things without a healer (you'd need to be overgeared, REALLY know your class, and overlevel'd most of the time). But that's just me. I do agree there are several different aspects that should go into consideration about your commendation. Communication is a HUGE factor to me, especially in the Hard Mode dungeons like Copperbell where teamwork is key. There would be times I ask "who has the bombs/crystals" and I would get replied by the DPS, and the tank just signs them away. Skill is kind of hard to catch though if you're not using a parser (which can be inaccurate). On a tank though, skill is a little bit more noticeable, you just have to tell if they're holding aggro well or not since that's their job (also, to not die c:). I tend to be biased to tanks as a healer, and healer as a tank or DPS. But there have been some DPS that have impressed me and sneak in my commendation.
It also depends if you're in a party with your FC! In dungeons, I sometimes drag two of my FC mates and the fourth person is a DF player. They get to reap all our commendations because heh, that's how the game works. I'm sure they're happy when it does happen, there's something oddly satisfying when you get all the commendations. (Story: a few days ago I joined a Garuda Extreme learning party and taught the WHOLE party their role, and got all seven commendations. I was so excited. I even got my ring. T'was great.)
It also depends if you're in a party with your FC! In dungeons, I sometimes drag two of my FC mates and the fourth person is a DF player. They get to reap all our commendations because heh, that's how the game works. I'm sure they're happy when it does happen, there's something oddly satisfying when you get all the commendations. (Story: a few days ago I joined a Garuda Extreme learning party and taught the WHOLE party their role, and got all seven commendations. I was so excited. I even got my ring. T'was great.)
"Critical fails; for when the GM sobs at night and the players get free checks."