I kept
Thrusting Bands for a long time because of the hit and expertise. It is missing on a good deal of the better tank pieces.
Some druids stack Stam. I have never been into Stam stacking. Before ToC I was all Agility. When they released the Shifting gems I started to use them in blue slots if I would get a decent socket bonus. Moving from Uld into ToC I have been going all epic Shifting ems to try and keep my health up while not gimping my dodge.
I like keeping my expertise as high as possible to reduce parry and also for the threat. Hit helps but not as much as expertise in my experience.
Now onto your questions.
1. Do I need to focus on Def at all?
No, it come on your non-leather pieces and more is better. But in general you are looking for high Sta, dodge, armor and expertise on those pieces.
2. What are the stat priorities.
Stam is king until you have enough health. Enough varies by instance. Naxx would be like 25k, Uld 30k and ToC pushing 40k. This is 10 mans btw. After health is at a decent place pile on Agility until it hurts. Armor on non-leather is something to watch for and grabbed when ever you can. At the same time you can't neglect expertise and hit. It will ***** you over. The crit and AP just comes with better gear. I tend to think of them as an after thought.
3. Does Def add anything? (Does it make sense to Gem or 'chant for it?)
Yes, Def adds dodge and to your chance to have mobs miss. Don't go out of your way for it though. Look for Armor, Dodge, Expertise and to a lesser extent Hit on non-leather stuff. The Def will typically be there any way.
4. As a Paly there was a hard/fast rule for entering Heroics - what are the baseline stats for a Bear?
I forget. When I hit 80 and now are two completely different games. What ever stats I had, which I forget, I would say you would have to 1.5 them. Sorry.
5. What is the big "Problem" for Bear tanks? For Paly it was always a lack of Stamina...
Threat not scaling as rapidly as with other tanks. We gain less per point of Str than other tanks since we just gain AP whereas other tanks gain Block and Parry. This can lead to threat issues if you don't get new weapons as fast as the DPS. I was starting to have threat issues before I got
Fordragon Blades.
OiN just wasn't cutting it in the threat department.
Otherwise problems are Rogues stealing my leather and Hunters taking my weapons.