Ok, lots of different directions. Relevant to this topic:
The bonus armor gear is a lot of fun, but not in line with this skewed semi-1-dimensional experiment. =) Trading avoidance for everything else is the opposite of what I'm going for. If you want to balance things, those pieces have fantastic EH values, so one or two could off-set more avoidance-centric gear and give you a more rounded set.
Bear in mind, this is a fun exercise, *not* a way I'm advocating as being the "way to play." I want to test something that is generally dismissed as a bad idea, and taking it to a real extreme to see what happens.
I used the most recent version of Rawr at the time this was posted. I don't use Rawr for predictive modeling, only to do the grunt work on arithmetic to add up all the stats, add buffs and talents, and give final numbers, rather than having to recalculate it all by hand.
40k health is not low for Marrowgar, per se. The measure of health *required* for a boss has a lot to do with your expected burst size. Marrowgar hits for several rapid-fire small hits. If you're taking hits big enough to threaten you in a couple swings, you won't easily survive it even if you have 60k health as it is *really* severe on the healers. Avoidance is applicable on all of Marrowgar's tank damage except for Coldflames which you should be stepping out of anyway. So, in theory, avoidance could be very powerful on Marrowgar.
Avoidance is not multiplicative (or multiplicative remainders like mitigation), it is additive. WoW uses a "Attack Table" or "Hit Table" that is taken up by all the various possible basic outcomes of an attack: miss, dodge, parry, glancing blow (only players against npc targets 3+ levels higher), crushing blows (only done by npc attackers 4+ levels above the player they're attacking), blocks, and hits.
So, your total avoidance is miss + dodge + parry. Miss is not represented on your character sheet, but if you have around 540 def skill you can expect an actual functional miss value of about 6%, 9% with Frigid Dreadplate.
More on that here: http://www.tankspot.com/forums/f63/3...bat-table.html
The major reason I am Blood primarily, is because I love the play style which includes a heavy, active roll in my survival. 90% of survival for most tank class/specs is in preparation and 10% is using your defensive abilities at the right time. As Blood it *can* be more like 25-35% active abilities.
You should stick with what feels best to you though, and if you like the Frost style, it may be in your best interests to find a way to make it work, to feel comfortable tanking with it.