diablo-3
Since the 2.3.0 patch, any rift keystones now have a message showing in the tooltip saying "This is useless. Perhaps you should sell it."
Given that they are no longer used to open the Nephalem rifts, is that indeed the case, or is there still some hidden use for them?
Watch the auction house.
Right now, crafting materials are selling for substantially less gold than all but the worst low level blue items. You'll get far better results selling your blue items and buying Essences from the AH.
For example, current market value for normal mode mats is 20g for Subtle Essence and 45g for Fallen Teeth. So any item worth more than 25g, I sell. If its worth less than 25, I gamble for a shot at a tooth.
Conversation availability does not change between difficulties, but some conversations are only available for brief periods (esp. with the jeweler and blacksmith). You can load these levels and get them immediately, though. For what it's worth, those A4Q1 conversations above were the most elusive of the lot.
Best Answer
Not that I know of. The purpose for the devs to change the Obelisk to not use key stones for rifts was to change the dynamic between bounties and rifts. It's doubtful that these have anymore use, I've gotten rid of my own since they take up stash space.