A warrior is the primary choice for (WoW Gold)maintank. He takes the least damage, requires the smallest amount of healing has the most tools at his disposal to ensure that he does not die.
The threat is awesome on trashes, not so good on bosses. Less mitigation and health = Instalose. Your multitanking ability is once again awesome on trash.
A paladin is the primary choice for (Cheap WoW Gold)offtanking and trash. Paladins have great utility when offtanking, supplying blessings and judgments on mobs and being able to spot-heal. When they gear up for mana/5 their healing becomes very competitive - With a well geared protection paladin capable of healing to an equal level as fully specced healers. Their healing ability compared to the other healers is closer than a druid's DPS ability compared to other DPS classes.
They are capable of the highest sustained health pool (A warrior can pop last stand and trinkets and get higher, but it won't be for long :-p) and can become immune to crits. Also, the armor potential they have makes them the only class capable of achieving 70%+ pure damage reduction. While they will never be immune to crushing blows, they are the only "tanking class" useful outside of tanking while wearing their tank gear. This becomes especially aparrent when you will attempt Magtheridon - If you have a feral druid tank on each add, after their add is dead they have 10000 times the utility in their tanking gear than a prot warrior or prot paladin.
Of course paladins are "viable" main tanks, it just so happens that we're the "least viable" as it stands at the moment.
And as for your constant comments about paladin threat - On a boss a competent warrior is capable of out-aggroing the DPS. When the boss dies, additional threat isn't magically converted into additional loot and if you're more likely to be killed without the healers being able to react (Yes, it will happen and a lot as a paladin) then your excess threat gen was in vain.
All I'm saying is, past Karazhan you're no longer going to the UBRS. There is no Molten Core, SSC is the BWL of Burning Crusade and the The Eye is the Naxxramas of Burning Crusade. There are instances that go even further than that. If Gruul is the Onyxia of Burning Crusade, then Magtheridon is Azuregos and all 4 Emerald Dragons all rolled into one.
You are a viable Karazhan tank (Albeit the least viable) but compared to a warrior in equivalent gear, you are not viable as an effective progression-oriented tank past Karazhan. Yes, you CAN tank every raid out there. Saying(Gold WoW) the opposite would just be silly. But it would take your guild 5 times as long to progress.