The is no repetition of old zones now. No Merciless etc.
There are acts 1-10. At level 67, you are the right level for Act 10. Now there was supposed to be a mechanism for making old, high-level characters act as if they’d finished all 10 acts, but I wonder if that kicked in at 68, rather than 67.
There are 4 Labyrinths of increasing difficulty - finishing them gives you points to put in “Ascendancy Classes”, which can really boost your build. I dunno if they’ll be automatically accessible for you, but if not you have to complete the trials to unlock them (the wiki can explain).
The Masters are people who give you missions, which if you complete them, give you favour with them. They allow you to build hideouts, modify items, and so on.
Then there’s the whole Atlas system at max level, which replaces the old map system.
Re: drop rates, well, they may drop less, I don’t recall, but it’s also possible the game now includes a “loot filter” by default. Certainly almost all players use one now - basically it hides irrelevant loot, and highlights particularly valuable loot.
Also, if you’re too high level, you don’t get many currency drops from an area. If you’re level 67, you’ll have a GIGANTIC penalty to currency drops in any area below level 50-something. So that probably factors in too.
I mean, basically, the entire game has changed massively. Personally what I’d suggest is, unless you really hate leveling, start a new character, and learn as you go. There’s a whole system that introduces concepts to the player as you level up now.