It comes down to gearing the toon correctly.

If you read decent tanking forums, for example http://maintankadin.failsafedesign.com/ (particularly for pallies)....gearing makes such a difference.
For example with my loladin....geared up pretty well, 540 def and a LOT of dodge, block and parry along with different gear sets for each.
Can go into heroic trash fights and even BOSS fights and not EVEN get hit....it makes a MASSIVE difference. MASSIVE.

Bear in mind, my toon is geared PRE Naxx. Avoidance and mitigation gear makes all the difference.

To be honest, gearing your tank should be a priority. It can make it so much easier.

I dont mean to pyss in your pocket at all, but one piece of gear wont make all the difference. It comes down to the whole package. Over time it becomes obvious.