"Tanks should be rocking close to 100k hit points natively through gear."

That's just gear; it doesn't include enchants, gems, food, or self-buffs. After those are applied a tank should be in the 125k-160k range.

However, I do fully agree with your mitigation point. That would be a nice guideline too.

I agree. I didn't venture into heroics (as a DK tank) until my iLevel was 335, and I had 130K hit points with only self-buffs.
And even then it was rough at times.

Though healer quality also fluctuates. I've tanked one heroic where the healer stated he couldn't keep me up, and then the next heroic the (different) healer ran the instance at full mana and said I was easy as cake (or pie if you happen to be of that persuasion) to heal.

But as a tank, I definately held back a little and didn't jump into heroics straight at the 329 ilevel average. Heroics are hard, and chances are pretty damn good that 2 out of the other 4 people are not ready for the instance you are doing with them. I've seen anything from healers with less than 60K mana (and running out after each and every fight) to DPS doing 4K DPS. You are why there's a tank shortage people. We don't enjoy running with you. Go back to doing regulars and get some real gear and improve your skill.

I'm sitting at 120k on my tank (still in a lot of i 318 gear), and would DEFINITLEY not feel comfortable going into a heroic based on what I've seen from the back of the group as my 346 geared healer. Tanks should be aiming closer to 150 than 120 on HP, and by my best guess, 25% dodge + parry would be a great start. Otherwise, you're just a mana sponge.

And healers hate mana sponges.

He said 100k hp through items alone. As in, your total hp minus your hp with no gear on.

Not 100k total. My priest in quest gear had over 100k total right at dinging 84-85.

Read things a bit closer next time.