A few notes on gearing, then, for tanking rated Battlegrounds. I doubt that many people are picking a tank main and off talent set. So how do we gear quickly and efficiently for both dps and tanking roles? Pick item slots to swap first that give you the most bang for your buck. I've picked trinkets first. Trinkets offer the largest difference between dps set and tank set stats. Next consult your bonuses from crafting and enchants to find the biggest stat swaps possible. If I'm a jewelcrafter and want to place agility jc gems in my dps set and stam jc gems in my tank set it would be best to place them in the gear that can also have the best enchant difference made. I can place them in a leg item so when I swap dps to tank gem items I also have a dps leg enchant to tank enchant swap as well. I can rock gloves the same way to swap engineering armor to dps enchants as well. Together I can have two trinkets and two items of clothing that between the 4 of them swap a ton of dps to stam and resilience.
Understand your mastery and how to activate it. My druid can take a lot of punishment but if I don't attack I lose out on absorbs and self healing from talents.
Understand where your mitigation works and doesn't. Dodge of parry aren't going to affect casters but resilience will. With base mitigation plus resilience you can hit some outrageous numbers. Recall that you need to face the enemy to dodge, etc as well.
Whatever pieces of gear you add to your swapping of sets are pieces you're going to have to farm twice each season.
Cooldowns that are pure mitigation are best combined with other mitigation cooldowns in play if needed as they compound another well. Some cooldowns are calculated as a percent of health, etc and can be combined with cooldowns that increase health for added benefit by increasing your total health, and thereby the health received by each percentage.