A hit/stamina gem would also be an attractive gem, but armor and block gems would be a godsend.

I agree that they would be great to have. However, Blizzard might be a little wary of the possibility of players loading up to the brim with block rating gems and effectively making themselves invincible in any scenario where damage per hit is less than the total block value (which would be pretty much any non-heroic scenario).

Of course, paladins already manage this to a certain extent.

Most tanks are already at this position regarding block value with any outdoor-physical mobs. For entertainment I went to kill some elites in skettis and in SMV (trees/giants respectively). In full tank gear with imp thunderclap / imp demo and shield block, the mobs were unable to damage me at all. Yeah, sure it takes 10 minutes to kill them, but I'm literally immune to their damage.

I got bored this past weekend and soloed most of SH. I think it's safe to say that invincibility in trivial content is an inevitable outcome of good tanking itemization.

However, I'm not so sure block rating/value gems would constitute good tanking itemization. Warriors would be able to gear out for passive crush immunity as early as T4. They've hinted at the possibility of removing crushing blows, though, so the 'worst-case-scenario' might not be any worse than what current raid gear allows for.