My first question is what do you plan to be raiding? If it's Naxx 10, you are very close to being ready. Drop the Strength gem from the gloves so that you can get to 540 Defense. Actually, you may want to consider dropping all the gems that have strength on them, and replace them with Defense, avoidance (Dodge), or Expertise. Strength is good if you are an add tank for the block, but it does little good when your the MT.
Overall, you've put together a decent starting set. Of course there are many upgrades to be had, but that's why you're looking to raid. Your number one priority is to get to 540 Defense. With out being there, you can be crit by raid bosses, which we can all agree is a bad thing.