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.
Agree about the dang camera with Soa. Grrrr.
Guardians just don't have much AoE threat abilities. We have our aoe taunt, force sweep, and cone attack, all of which generate extra threat, and Guardian Slash, which not many people take and only does aoe situationally. I feel like I do a pretty good job with the tools I have - I can generally hold a decent pack of mobs. But some players think if a single strong is not attacking the tank or goes for the healer, it's the tank's fault. Well, that guy was probably standing off to the side a little and should have been cc'ed or burned by dps as a priority target.
Guardians should have absolutely no problem holding boss aggro. There is no excuse here. If your group is competent and giving you 3 seconds to establish threat, and using their aggro dump after their initial burst, you should not lose threat. If you are outgeared by dps, you may lose it once, and only once, at the beginning. After you taunt back, it shouldn't happen again.