I would say the biggest factors that most people use to define a great tank is:

1. Ability to keep aggro. No loose enemies here and there.

2. Ability to rotate tanking cooldowns, when neccessary. (mainly noticed by healers, especially if the tank is not over gearing content and/or is doing huge pulls)

3. Understanding what makes it easier for certain dps roles to do their job, i.e not rotating enemies when you have monks/dragoons with you to make it easier for them to hit the positional requirements on certain skills.

4. Understanding and remembering boss mechanics.

If you meet all of those "requirements" you're mostlikely going to be "a great tank" in Duty Finder partymember's eyes.

These are based on personal experience as tank and healer and dps from countless DF parties.

Note: I might have not remembered something which is really essential and comes as a automated action of which I don't have to think about it to do it.