A few years ago I gently poked fun at Cryptic for wanting to fit all the game's zone quests into their "campaign" endgame model. Considering what an awkward fit that was, I like how they've split content into campaigns and "adventures" with mod 21. A campaign is still what it always was: a grind to repeat content like dailies or heroic encounters in order to fill a bar and earn things like boons and access to higher-level group content.
An adventure on the other hand is simply a series of story quests - whether inside a single zone or delivered via loosely connected solo instances - that you're only expected to complete once, and at the end you get some more mundane (but in the new system also quite high-level) rewards such as increasing amounts of AD, gear, plus conveniences like mounts or bags.
All the old levelling zones that were kept are now adventures, as is Acquisitions Incorporated, which was always terrible as a campaign, seeing how it was basically a collection of story quests that the campaign framework required you to grind over and over again. The Underdark story (which was always completely disconnected from the campaign with the same name) has also been reclassified as an adventure, and the associated campaign to grind heroic encounters has been completely removed. This does make things more consistent.
The only downside from my point of view is that since they upped the rewards for adventures so drastically, they kind of feel mandatory to do on all characters now. I used to skip a lot of the old zones while levelling up my alts just to keep things varied, meaning that I'm now looking at potentially re-running zones like Neverdeath Graveyard, Vellosk etc. on half a dozen max-level alts just because the new rewards are too good to pass up.