The answer is NEVER diminishing returns.

If anything, the opposite should be true. Diminishing returns go against everything the Cataclysm rediesign was supposed to do. "We made it so you don't need complex math to figure out how much of something you get based on your rating. Your character sheet tells you." (Unless you are a tank.)

Diminishing Returns should be taken out back and shot in the head. All ratings should be what they say. All tank gear should have 3 stats. 2 survival, 1 threat. Remove the auto-hit from taunts. That's right. remove it, make all tanks need to cap normal hit to cap taunt as well and suddenly the gear becomes a bit more reasonable. Make a missed hit matter. Additionally, remove the extra parry chance in the front arc a mob has and make tanks want to cap expertise. Once those are capped, it should be a little more reasonable for tanks to gear.

As for block, make the mastery give a lot less of it, but make it scale the amount blocked as well. So if you do cap, it is still beneficial to keep going.

Passry should be changed to be 50% damage from that and the next hit. It should stack so that parry streaks don't make you lose mitigation. Bears should also get it. Bear mobs can parry, heck it isn't like 8 inch claws can't be used to stop attacks, rogues can parry with daggers. So now every class has dodge, parry and mastery to stop damage. Monks also getting Parry would help. Heck, make them both use talents that make haste increase parry. Make crit help their mastery and you have them wanting all the stats too, just like paladins and warriors for their gear.

The blizzard team is too unwilling to make big changes when they can. Changes that are needed.