The Marauder set is a trap really, it's the "only" good setup for Daemonhunters wanting to try the higher difficulties atm. I don't like the "plant a turret and Tumble behind a wall" playstyle with an extreme glasscannon build that it promotes...
Too bad for the low-end stat rolls though this is what I meant by the "steep slope" you need to gather dozens of the same pieces in hopes of getting high stat rolls on stuff... at least you can squeeze a bit more out of that piece too by putting bigger (more upgraded) gems on that thing (280 is the max per gem). It's a min-maxers nightmare! If you do get some trial keystones for Greater Rifts, try and run a few of them to get the Legendary Gem that increases Pet Damage as well (though they require you to make hard decisions on which stats to re-roll in a ring or an amulet to make room for the gem...)
Normal Rifts have been in the game since RoS came out, and they're where I usually gather my legendaries and set items as they have around double the drop rate there compared to regular bounties or campaign play You also get a lot more Blood Shards to spend on Kadala for gambling compared to the few that drop from Bounty Caches.
Greater Rifts is a more recent addition, with special trial keys dropping in regular Rifts that allow you to enter Greater Rifts after a small "DPS-check instance" that determines your entry level. You have 15 minutes to complete each level and if you do manage to kill the Rift Guardian on time you get to advance further in the GRift, if not you only get an option to level up one of several Legendary Gems that are an exclusive drop from the GRifts which slot only into Rings and Amulets.
Have you tried hunting for the Keywardens in Act 1-4? they drop special key parts that can be crafted into portal items that open up an über-boss instance in New Tristram which drops items that allow for crafting Hellfire Rings and Amulets, the latter of which is a fairly new addition that holds a random Class Passive skill as one of it's properties.
I think the first season is a bit long though. I've gathered more Paragon levels during this first season than during all my regular play since when RoS came out I like the changes to 2-handers and have found Barbarian and Monk to be fun to play again in the season.
I wouldn't really call D3 vertical progression either, more like a steep slope since you can get Torment 6 (max. difficulty) gear fairly easy, but max stats are very grindy and fully RNG based to acquire, and not really necessary either unless you want to reach those top spots on the leaderboard for Greater Rifts, and even then it's more based on the RNG on enemy composition and map layout...