I get asked this a lot. It's frustrating.
Gear progression doesn't have checkpoints or milestones. There's no "I have X hit points and Y armor now, so I can tank boss Z." You can be undergeared, you can be overgeared, but there's a huge area in the middle where you're neither - you're just "geared". Whether or not you're geared adequately depends on a wide variety of factors. How are your healers? How is your DPS?
The fact is, every tank - just like any other WoW player - should always be looking to upgrade, until you have the absolute best items available to you. There's no stopping point before then. So stop asking me or anyone else for one.
Tell your friends,