Here's an example of the problem I have with paladin tanking gear, and what actually prompted me to make my last post.
I have two tanking boots: [Flesh Beast's Metal Greaves], from Mana Tombs; and [Boots of Righteous Fortitude], from Heroic Sethekk Halls.
[Boots of Righteous Fortitude] are gorgeous tanking boots, and above all they are paladin tanking boots. Yet if I was going to tank Karazhan, I'd wear [Flesh Beast's Metal Greaves] because they bring me much closer to uncrushability.
This seems wrong to me. Game mechanics should not make me choose an almost total downgrade. If the mechanics do so, the mechanics are flawed.
You can see the same problem with Righteous Armor. It's actually a really good tanking set, except that it fails the uncrushability test. If paladins didn't need to become uncrushable, there's a decent argument that Righteous Armor would be the paladin tanking set of choice for a raider just starting Karazhan.
(Well, there might be better pieces overall, but people wouldn't laugh at you for trying to tank in it. Unless you're in the Raid and Dungeons Forums. Then you'd get laughed at for not having epics.)