Hi Alex, I'm actually really glad to see you posting here. This gives us a chance to discuss some of the points you've bought up.
I think there's enough comments and information in the replies on this topic that that I don't need to dive into the talent tree as well as the nitty-gritties (and embarass myself talking theorycraft), the rest board have done a good job on that already.
Prot Paladins, like druids and warriors, has different strength and weakness dealing with each boss. You can see the hard work done by members of this board, analyzing the boss in game in regards to Prot Pally: Cordelia's Raid level boss guide
http://www.failsafedesign.com/maintankadin/viewtopic.php?t=3619. The information is outdated because the original thread creator is gone, but you can fill in the blankets on rest of the stuff by going into the corresponding forums (Mt. Hyjal, Black Temple). Shear done by Illidan is more reliably avoided using holy shield.
Dealing with BT from Wordie's post on theoryspot.com: "
Also i would like to remind you how Paladins can be considered superior tanks for the following fights of Black Temple.
1) Supremus (ranged pickup after threat reset)
2) Illidari Council -> Rogue. Since we can taunt him if he goes spell immune, avenger shield him if he goes melee immune.
3) RoS p3 (and p1 by some degrees) are more effectively tanked by Paladins as well. The damage taken from Soul Scream is trivial compared to the potential damage burst on warrior in case of 2 unavoided hits, and threat generation scales way better than warrior/druid's one with the Aura.
4) Shade of Akama, capable of AoEtanking the groups of 3 adds without too much issue.
5) Bloodboil, a paladin in the tanking squad for him can provide a additional BoP for a tank that goes too much on with aggro, as well as bubble himself in case of getting too many debuffs.
And about Illidan, paladins can tank both the boss himself or the flames, according to how the guild prefer. By tanking the boss, you give up some mitigation and emergency buttons for snap threat after phase transitions and a more reliable way to avoid Shear. By tanking the flames, he can bubble out of the Dark Barrage in case he happens to get one, avoiding a very potential wipe."
There are many, many guilds that have down Illidan that uses a Paladin as main tank. Off the top of my head, <Lunacy> on Proudmore, <Primals> on Ravenholdt, as well as many of the MTs that prawls this forum.
So if a protection paladin has the same capabilities as other tanks, why do people still have the idea that Paladins cannot be main tanks? The only thing I could come up with was:
prejudice. The stereotype have been ingrain into people's minds, and I believe this is what is so frustrating to the people here. Each and every one of the protection paladins I know works hard to get rid of this prejudice. That's why people are frustrated when they see a reputable blog like WOWinsider not understanding the stigma we've been fighting.
I hope this helps.