You are a bad tank if:
You do not have "Target of Target" enabled in your UI.
You fail to hold aggro for one or more mobs.
Test case: Kaon Under Siege
You fail to hold a boss's aggro throughout most or all of the fight.
You are a good tank if:
You can place an enemy exactly where they are supposed to be.
Test case: Soa
You can area taunt with precision.
You coordinate tank switches efficiently with your other tank.
Test case: Fabricator, Zorn & Toth, Ciphas/Heirad/Kel'sara
You use your defensive cooldowns effectively
I'm not sure I totally agree with everything you mentioned about the Tank spec.
First off, Soa is a bad example to determine if someone is a bad tank. The camera angle in game isnt that great for trying to pick up the pillars that appear during the fight, so it can be difficult for a Tank to maneuver Soa into position under the spinning pillar.
I also disagree on the Holding Aggro piece. Some tanks, and Guardian Tanks in particular, from time to time have difficulty holding Aggro on Mobs. Not really sure why this is, cause most other Tank specs have little to no trouble holding them. I didn't believe this until my Guardian had similar issues. Now I just drive my Threat up and that usually takes care of that problem.
This of course leads in to holding Aggro on Bosses, which Guardians also have issues with. The use of Taunt and Challenging Call are almost necessary to acquire or maintain Aggro for most of the fight. With Guardians, the DPS might want to be careful what they use during the fight because raising their Threat level can pull the Mobs and/or Boss off of the Guardian Tank, and if that Tank is waiting for Cooldown on Taunt or Challenging Call, that DPS could be in BIG trouble.
I can't speak to experience on other Tanks, but the Guardian is a unique specimen.