Some useful info from Blizzard on Greater Rift Difficulty and Rewards. The first comes from Wyatt Cheng, following up his Monk Exploding Palm GR100 reply:
Wyatt Cheng: The reward scheme for Greater Rifts is still under heavy iteration at this time. We are primarily concerned with the gameplay mechanics: acquiring a key, progressing through the rifts, difficulty tuning, multiplayer dyanmics, etc.
The reward needs to be proportional to the effort and time invested. With the mechanics of Rifts changing this is a moving target. In the next PTR patch you’ll see more of the Legendary Gem reward mechanics implemented. The PTR patch after that we’ll start dialing in the reward structure.
Please feel free to give feedback on what you see, but please know we are aware that the reward is off right now. Our goal is for Greater Rifts to be absolutely worth doing, and you should always be trying to clearly quickly towards the highest tiered rift you are capable of – but we’re not there yet.
Incidentally, though the GR difficulty isn’t tied to your game when you enter the GR, all GRs can drop Torment-only gear. As I pointed out in a GR general info forum thread, I got Akkhan’s gloves from the GRG on my first level 1 rift, which is equivalent to Normal (or less) difficulty.
Speaking of Greater Rift difficulty:
Nevalistis: There isn’t a direct correlation between Greater Rift difficulty and the usual difficulty modes. They are independent from one another, though you can make rough comparisons through your own experimentation.
However, we do plan on tweaking the difficulty scaling in the next PTR patch, so you may find those lines different from what you’re experiencing now and perhaps a bit easier to draw. We really appreciate the feedback we’ve received on Greater Rifts so far, and we look forward to seeing your thoughts in the next PTR patch!
Our Diablowiki.net Greater Rifts article is massive and info-dense, and while Nevalistis is correct in that difficulties can not be directly equated, we’ve approximated a fairly precise scale based on player testing. (Until the next PTR patch changes it all, of course.)
Those are just estimates, of course, since things differ so much from GR to GR. There’s at least a level or two of +/- of variance. You might well get a level 20 that feels easier than 19 or 18 did, depending on the spawn. I’ve seen video of players barely scraping through a level 23, and then finishing 24 with 6 minutes to spare when they got easier enemies with higher density. Expect variety, which is no doubt Blizzard’s goal. RNG in all things.