Well the obvious issue is you're below crit cap. You should never rely on a the sigil to get you over 540, what happens if you get silenced like when tanking Anub'Rekan ? Replace that hit gem with a defense gem, and work on your SoH rep for the shoulder enchant and you should be over the line.
Spec looks good other than taking Mobidity over Epidemic, but that's a playstyle choice.
Remove the Defense gems from your gear and put the 16 defense enchant on your cloak for starters. Then you can swap in your choice of hit/expertise/stam gems as you are still lacking in that area. Your Hit rating especially is very low for a tank. Again, you can replace that Defense Sigil with the Sigil of Awareness as well. Its only 25 Valor badges, and it is quite a nice upgrade to your threat as opposed to 10 more defense when you are already capped.