OK- this is coming from a newbie tank. In the other MMO, I usually played a caster-dps role, but never tanked. I was familiar with what a tank did in the other game, and was always fascinated by it. I started playing this game with my wife. She suggested that we become a tank/healer team. Basically I learned to be a tank in TOR.
I do admit that the Guardian could use a little help with the threat problem. BUT with practice and realization that this tank isn't the easiest class to use, I am able to hold threat without a problem.
I am specd full defense. If you are having issues keeping threat, there is on place you can practice until you get used to it: Open Communication – Heroic 4 on Belsavis. It's a 4-man Heroic, but can easily be done by 2 people, especially a Tank/Healer team. Being a first time tank- I learned how to actually hold aggro using this heroic. I understand that this heroic is just one example of the many different types of encounters you will face in an actual FP- but it has worked wonders for me in actually understanding how this tank works.
I think the players who have tanked before in other games are expecting this tank to be the same. I wouldn't know...but I know I am not experiencing the same threat issues as others here who have tanked before in other games.
Just my 2 cents...