Your health pool seems alright for now. You should look at getting your avoidance stats up, however.
Remember that a large health pool is meaningless without proper avoidance to back it up. A large health pool is designed to make the blows you do not avoid less painful, and not to absorb hit after hit after hit.
You may also want to look at getting Crygil's Discarded Plate Panel from the Heroic Badge vendor. It gives you Dodge and Hit over the raw Defense on the Titansteel shield. However, be prepared to lose a large chunk of Defense.
I recommend getting Seal of the Pantheon or Repelling Charge to allow you flexibility in gemming (with either of those, you shouldn't need to gem for any defense).
Also, with your Talents, take 5/5 Shield Specialization. I'd probably reccomend dropping the 3 points in Iron Will to do this - Stun duration talents are nice, but are situational in fights where stuns are intensive. 5/5 Shield Spec. will give you all-round increased avoidance, which is much nicer.
Hope some of this helps, let me know if you have any questions.
I disagree with your suggestion to put more points into shield specialization. I find that chance to block is a poor thing to improve. Dropping iron will means no more impale/dw which destroys my threat generation.
A 20% stun/charm duration will not provide sufficient relief from any major stuns in high encounters. Most stuns against a Tank last for a short amount of time - I have never encountered one that incapacitates me for so long that DPS can breach my threat. For fights that do have such Stuns, there is a usually a strategy in place to prevent the above situation (OT pickup etc)
You also said you were fine with threat.
You asked if you needed to work more on Health or Avoidance - my answer is Avoidance. You're probably alright for 10 man Naxx but anything beyond that you might have trouble.
Also, could you explain to me why chance to Block is a poor thing to improve?