They've made it so that you can complete the quests in the zones in any order, which is nice. BUT the quest lines themselves are very standardized, linear, and predictable. I will always miss Vanilla when the quests were much more varied in the depth, rewards, etc. Basically, part of the game back then was using intuition to choose between quests or zones to focus on/complete. But WoW has been sticking to the linear model since tBC, so whatever. For the record, Legion has relatively fun linear quests.
Thinking this through again. The majority of quests of what can be construed as the main story line is mostly linear until you get to Suramar. There still are dozens upon dozens of pocket quests and flavor story lines that have no breadcrumb trail. These are stumble upon quests and are reminiscent of Vanilla and are found through straight up exploring.
Haven't done Suramar yet but I've heard good things about it. Otherwise, I see what you're saying... although there's another element that's hard to put into words but I'll try.
I've come across some of those quest lines that you talk about with "no breadcrumb trail", but it's not as if I explored to find such a quest. I walk right into it because it pops on my mini-map as I navigate the main quests. And that is one of my complaints with these zones, they are so tightly packed with quest zones, rares, treasures, that you really can't help but inadvertently encounter them. Sometimes I feel almost claustrophobic; Last night while I was accepting (reading) a quest from the quest giver, there were some horde fighting a rare NPC right on top of me. They basically just pulled it from the pond about 10 feet away. It's almost as if these zones are designed as if we're all suffering from ADD and about to lose interest at any moment, so they throw everything in our faces. The ultimate effect is that when I do kill the rare, grab the treasure, find the quest, I don't feel any satisfaction of having explored, or even chose that course of action. Does that make sense? When I think of Vanilla questing (pre-Cata), I remember some large open zones with plenty of essentially unused space (think of Tanaris or the Wetlands). Zones such as that were definitely my favorite and made the world feel natural. Unnatural is probably the best word to describe what I don't like about the Legion zones.
Even the design of the quests have changed a lot in that respect. For example, some Vanilla quests were absurdly difficult or took way too long for the reward. Now, they all take almost exactly the same amount of time and require the same amount of effort. Also in Vanilla, quest premises were significantly more modest, even at max level. Nowadays, I'm constantly saving the world (along with 50 people around me simultaneously saving the world). But of course while saving the world, sometimes I get locked in a wooden cage only to be saved seconds before certain death. I'd almost prefer if they (quest givers) would just say hey, if you get a bunch of the pelt from this deer I will make you a piece of gear, and that's it - no continuation. Is that objectively "fun"? No. But at least it makes the world feel natural. Not contrived to the point where it feels like each quest line is a short screenplay written by a 6th grader. Then, I would have more "fun" overall progressing my character through a world that feels somewhat real.
But perhaps if Blizzard took my advice, half the player base would immediately quit out of boredom.