The Avoidance vs Health debate for tanks is eternal, it seems. It seems to me that you should have a balance of both. For starting heroics I figure a bear should have 25K in both health and armor and 35% dodge. Stack Stam and Agility until you reach these numbers. After that you should stack Stamina until you get to 30K health. This is an important number as many groups won't take a tank with less health than that.
After you have 30K health I recommend you stack Agility until you get your dodge up to around 40%. Agility also helps your armor rating so you should have plenty of avoidance/mitigation at that point and can stack Stamina from there. Our guild is only in Naxx but so far I havent found any downside from the stam cram. Supposedly in the Ulduar hard modes Stam is even more important.
With TEH GEAR SCORE! being all the rage lately I think health is not as large a factor as before. Assuming this continues, and they improve the gear scoring addons, then having the best gear will be more important than your health. Before I was recommending to bears that they grab the polar gear (chest, belt, and boots) and cram them with full Stamina (after you are no longer using them for regular tanking runs) so you can wear them when sitting around in LFG.
People are much more likely to take you when they see that huge health pool and the "vanity armor" does have some uses. Supposedly its good for tanking Hodir. Our guild isnt there yet. I have used to to offtank Patchwerk (taking the Hateful Strike) and it works for taking down that pesky mage boss in Occulus.
