Ok, here's my take on it. The other tank needs to do two things. The first, they need to know wtf they're talking about. You've got plenty of gear by both gear score and any other measurement you might want to bring into play. The second is that if the raid wipes with you tanking the boss, maybe the other guy needs to trade places with you.
Some things you might look into:
Plain and simple, more strength. You might not be having threat issues, but more strength will also up your avoidance due to a DK's passive "Forceful Deflection" which turns strength to parry. Not to mention you'll have better threat, which is never a bad idea. Also, Marrowstrike is a damn good weapon. Don't get it twisted, it's great. For a hunter. It'll do you solid until you get something with strength on it instead of AP, but when that option presents itself, dive on it so long as it's not too big of a damage downgrade. DKs get exactly jack out of agility, as opposed to hunters who get double ranged AP from agility, much like DKs do with strength in a melee capacity.
...Thoughts of a philosophical Death Knight...
If death is the inevitable end of life, is life the inevitable end of undeath?
Depends on which strategy you use. One employs taking out a construct at a time, the other just has you pick up each add as they spawn and tank them all. The latter requires well geared tanks because the stacking debuff will make it impossible to heal both tanks in short order. It's an all out DPS burn from start to finish.
If you are on adds and use the one at a time approach, the adds hit for nothing, it's the fire damage that you take while kiting them through the fire that will kill you if you don't mitigate the damage in some way. This is why you usually have a class that can slow or lock down the add in the fire while not actually in the fire themself. Have someone root the construct in the fire, chains of ice them in the fire, what have you. The idea is that the construct stays in the fire, you don't. If you rotate OTs on the adds, there is less damage per OT than if just one OT does the adds.
To be quite honest, both tasks are relatively easy to do, the OT job requires more movement than the MT job, that's pretty much it.
As for parry, it is the one stat that has the highest diminishing returns for a DK because of our racial. I would never socket or itemize for parry at all. If the gear comes with it, fine, but I would never go out of my way to acquire it. Dodge is always a better stat to work towards. Stam of course being a no-brainer.
Well since posting my OP I have replaced the 3 blue items I had and replaced my HoL trinket with Essence of Gossomar. The only way I can get upgrades now is by doing Ulduar or getting badges of triumph (and perhaps a few VoA runs, but nothing terribly exciting in there)
I think you gear is fine. The combo of your gear and your tanking spec though might not be the best. Blood tanking doesn't have the damage mitigation that the other specs have so it relies on a larger health pool. The health pool + Vampiric Blood + self healing help the Blood tank a lot. I have the same issue so I went to Frost and then Unholy tanking and I did better in both specs. I settled on unholy though for is extra magic mitigation as my raid has plenty of tanks that have high physical attack mitigation.
Yeah, your gear looks fine to Uld10 to me. Your group's makeup, buffs available, and the general skill level of the rest of your group can affect things greatly though. They may be accustomed to putting more emphasis on the tank's gear (a common theme) to overcome any shortcomings in the rest of the raid.
Or it may just be the "zomg u cant tank heroics with <30k hp" syndrome at work here.
As people have said, DKs are definitely great for tanking the adds on this fight - while it could be seen as a "harder" job (and to be fair, tanking adds is often the more complicated tanking role) DKs have a range of tools available to them that really help. Chains of Ice can keep them in the fire until they're almost molten, Anti-Magic Shell can be used for those times where you simply have to stand in the fire, Deathgrip can pull stray adds that get in the raid or pull Molten adds straight into a pool... and so on.
As a Paladin all he can really do is stun them for 10 seconds in the fire and... well, just stand in the fire and take it.
10-man Ulduar was tuned for people in full Naxx-10 gear and it's been nerfed since then! Now that you can get a range of 200-226 epics plus a 232-245 or two simply from running Heroics and nothing else, nobody should be realistically claiming you're "under-geared".
Sounds to me like he was probably struggling with add management due to his class not being optimal for the role and blaming it on you. Maybe the healers weren't getting silenced by Flame Jets but were having to spend too much of their time focussing on him standing in the fire?
When you guys get to Iron Council he'll probably say you should be on Steelbreaker because "tanking two bosses is harder". Sigh.
What Mert said. Pala & warrior tanks have to stand in the scorch so they can pick up the add after it goes molten & resets aggro (while the healers are aoe healing the raid, thus generating massive aggro). Tanking adds IS usually harder but not in this case as a DK. Getting the right man for the right job is what raid leading is about.