I usually give commendation to whoever I feel worked the hardest to make the run go smoothly.

The DPS who is always using cooldowns, dodging all the AoEs, using AoE attacks appropriately is likely to get a comm.
The tank who uses cooldowns, keeps mobs positioned well, pulls according to the group's strengths is likely to get a comm.
The healer who never lets anyone die or get so low HP the tank starts freaking out AND uses Cleric Stance to DPS is likely to get a comm.

If everyone is performing REALLY well, I will usually give it to the DPS or healer, depending on the instance.
Having a healer main job and DPS sub job, I've done every dungeon on both. I will give comm to whoever I feel the dungeon is more difficult for.
There are 3 reasons why I don't give to commendations to tanks as much as DPS or healers.
1.) I haven't leveled a tank class high enough to really know how well they are performing. I'm not in a position to judge accurately.
2.) I'm certain someone else will give a comm to the tank.
3.) The tank likely got a bonus for being the lacking role in the group and also likely got an instant queue.

However, there are some things which immediately disqualify a player for a commendation.
1.) Any rude / obnoxious comments or behavior is the biggest reason.
2.) Going /afk during the dungeon or chatting too much with other people not in the group. (It's clear when a mob dies and you just stand there spacing out while everyone else is attacking the next mob.)
3.) A DPS or healer who purposefully pulls mobs. I don't even play tank, and this annoys me. I know it can be beneficial to keep up rotation, but it bothers me when DPS is off rotation or buff already is gone.
4.) A healer who lets someone die, when it isn't their first time in the instance, and especially when the damage should have been healed.
5.) A tank who is spinning mobs around and moving around way too much
6.) A caster who uses limit break on the boss when it is above 15% HP and there is a melee DPS in group who could have done more damage instead. (Unless I am confused about the way LB damage works...)
7.) A healer or tank who tells the DPS when they should use limit break, especially if the boss has more than 15% HP. If you want to decide when DPS should use LB, join the dungeon as DPS.
8.) Any player who does not greet other players. (If you queue for Japanese group, you should at least know basic etiquette for that kind of group. It's very rude to say nothing.)

I never consider glamour when giving commendations.

Any player who gives advice to new players / first-timer is very likely to get comm.
Any player who asks for and follows advice is extremely likely to get comm, even if they didn't preform well at first. Making an effort to improve yourself and trying hard to perform better is very attractive quality!