One of the most exciting features of Legion, especially for altoholics and people who like to play starting midnight on launch night (both describe me,) is that the Broken Isles' four level-up zones are playable in any order. There will be no single set path through the continent, and zones will dynamically scale with you so that you're always in an area that is an appropriate challenge.
One implication is that this is going to mean far less bottlenecking - players won't pile into a single zone as the expansion launches like Hellfire Peninsula or the Jade Forest, instead they'll be spread across these different zones (after spending up to an hour doing their first artifact quest - a long enough time to stagger players a bit.)
Some parts of each zone, and all of Suramar, will be locked at level 110, meaning there will be dangerous parts of each zone that you'll want to avoid while you're leveling up. But once you hit 110, the entirety of each zone will be scaled to players of max level, meaning that Blizzard will then be able to put World Quests anywhere on the Broken Isles and still have it present a modest challenge.
The scaling system is on display in the demonic invasions that have been going on for the past few weeks. If you're level 15, the demons are roughly at your level (tougher ones being two levels higher.) They do damage you'd expect from a level 15 enemy, and a level 15 character can take them down as if they were the same level. Yet the very same demon, to a level 100 character, will appear to be level 100, and rather than hitting for 600 damage, it might strike you for 60,000 damage.
What this means is that if you have a friend who is leveling up slower or faster than you, you can still quest together or run dungeons together (the level-up dungeons will also have this tech) and each will feel as if the content is at a level that's appropriate to him or herself.
One could reasonably ask "why have levels at all, then?" Well, new profession quests will unlock as you go along, and they still want you to have to quest through the Broken Isles before you get to the new level cap. For one thing, they need to make Warlords gear obsolete and let secondary stats diminish so that we don't wind up with players having 100% crit and 200% versatility.
Perhaps this does make the distinction between level 102 and level 108 somewhat less meaningful - in a sense, you can almost think of the Broken Isles as a single area in which to level up once - from 100 to 110. Still, other games, like Skyrim, have had this kind of level scaling and I don't think anyone complained about that (ok, I'm sure someone did, because someone will always complain about anything.)
What I don't know is whether there's a level requirement for starting a new zone. After acquiring your artifact, you'll pick a zone to start off in. Given the way that this scaling works, there's no reason you couldn't take a break from Azsuna, head over to Stormheim, and then come back and finish Azsuna later, but I don't know if there's any sort of gating on when you get your starting breadcrumb quest.
Personally, my plan is to use a d4 (for those who don't play tabletop RPGs, a d4 is a 4-sided die) to determine which zones I go to on each character so that I don't develop a predictable pattern. I'm honestly pretty excited about all of the zones, so while my main, the paladin, is eager to go do Titan-associated things in Stormheim, I figure that can come whenever the dice tell me it should.
I do wonder if this sort of tech could be applied to older content. On one hand, that could be really great for making low-level zones that these days you outlevel before you're a quarter of the way through them more relevant, but the Broken Isles also have their stories built on the assumption that you're doing them in different orders. It would be weird to go do Talador and see Maraad die, only for you to go fight alongside him in Shadowmoon Valley.
Still, assuming it's a popular feature, I think we could see this as the future of WoW.