One of the features I'm really excited about in Legion is that we'll be able to choose the order of zones we level up in from 100-110. While I previously though that Suramar would be a level 108-110 zone, it appears that you won't even start questing there until hitting the level cap - and if it's really a questing zone with a full story that just expects you to be at that cap, I'm also pretty happy about that (the last time we had real outdoor solo questing designed for people at the level cap was probably Cataclysm.)
But this zone scaling feature could be a huge deal. For one thing, assuming it works (and I'd guess that was what that whole Beta test was for,) it should spread players out across the continent in the early parts of the expansion. With people heading off in different directions for their artifact quests, their class hall quests, and then choosing their own paths to level, I think we're going to see the exact opposite of the bottlenecking effect that the Tanaan Jungle intro had on Warlords of Draenor (that said, you can still expect DDOS attacks from trolls and a massive number of players logging in, so don't be shocked if it's still messy the first night. At least we'll already have our Demon Hunters!)
Let's take a look at the bigger picture though: assuming this feature works well, we're likely to see it in expansions going forward. The question, then, is if it could be applied retroactively.
Those of us who have been playing for a long time (ten years this fall for me) don't really need to level up new characters. Even if you aren't as big of an altoholic as I am (name of the blog,) if you're a WoW veteran, you've probably leveled up the classes you're actually interested in playing by now, and at worst, they're sitting at 85 or something, with only a few expansions' worth of questing needed to get caught up.
Add on to that the fact that Blizzard is giving us a level boost with each expansion and you get the distinct impression that Blizzard doesn't really want to worry too much about the older parts of the game. I highly doubt we'll ever see another Cataclysm-style revamp of the old world because, well, now they just let us skip it. And with Warlords and Legion, you'll be able to get a character to 90 and two to 100 without doing all that much in game (one of your two 100s will be a Demon Hunter.) So the big caveat here is that Blizzard's current strategy is more about sweeping the level 1-100 game under the rug rather than making it easier to quest through it.
But if there is a shift in philosophy (plenty of players want more than three characters,) this zone-scaling could really make leveling up a whole new experience.
Being able to go from Ashenvale directly to Felwood makes perfect sense geographically, and with this technology, you could do that easily. It would also allow you to differentiate zones that are currently the same level range. Let's say you want to do Valley of the Four Winds and follow it with Krasarang Wilds. Today, that would mean doing a bunch of quests that were falling behind on their rewards and getting a bunch of gear that is no better than what you've already got, but with a zone-scaling system, you could get through a big chunk of your high 80s that way.
Or, you could use this scaling system to change when you go to the various expansions' areas. Granted, this would screw up your timeline even more than doing zones in any order (though that would also happen with simple zone-to-zone choices,) but maybe when you hit level 58 - or hell, 40 - you decide you'd like to go to Pandaria, then bounce back to Northrend, take a brief visit to Outland, and then go to Draenor before dealing with Twilight's Hammer in the Cataclysm zones.
Given that the constant shrinking of XP requirements to level with each expansion, these days you often get a third of the way through a zone and start to feel that you're out leveling it. I think even when Cataclysm had just come out I couldn't get through Darkshore without some of the quests in my log going full grey.
Zone scaling would allow you to spend as much time as you want in any given zone and pursue the story. You might hit level 40 before you get out of Ashenvale, but that's fine if you're just a Night Elf who is very passionate about defending your home territory.
I do think some restrictions would be important. It's already weird enough for me that you can do Eastern Plaguelands before Badlands now (for those who started playing post-cataclysm, Badlands used to open up around level 35 whereas Eastern Plaguelands was an endgame zone that you didn't want to go to until about 55 and would have quests there well after you hit the original level cap of 60.) It would be pretty absurd for you to fight against the entire Dark Horde in Burning Steppes and then start worrying about Murlocs in Elwynn Forest.
It might even be important to restrict expansions to the level ranges they originally had - you'll spend your 60s in Outland, for example, even if you get to do Nagrand before you do Zangarmarsh.
Cool as this sounds, I suspect that the level-scaling tech is probably more likely to be used for future expansions. Most of us players don't really know what it's like, though I'm given to understand that most people who have played in the Beta say they sort of forget that it's a feature, which I think is a ringing endorsement.