If you were playing on launch day, or even within a few days of it, of Warlords of Draenor, you probably remember utter chaos. The servers were crashing and it was a real struggle to get things started. Every expansion has had this issue - people flood into the starting zone and there's massive kill-stealing and just lag from all the crap that's going on.
In the past, they've tried to separate things out through various methods to keep things from getting bottlenecked. Expansions in which we've had new races or classes have encouraged people to try the shiny new thing first before heading into the latest content. There have also been expansions where there are multiple starting zones, usually with Horde and Alliance towns that are rather far from each other.
Warlords had a smart idea in giving entire zones to each faction, giving them a lot of time separate from each other, and with multiple quest chains that could be completed in multiple orders. But everyone started by being funneled into the Tanaan Jungle intro, which meant that everyone still basically had to squeeze in through a very tiny gap to make it to where the road widened.
It didn't help that the only "new toon incentive" in Warlords was to boost a character up to 90 - meaning that even if people were trying something new (and beyond existing race/class combos, there wasn't anything really new to try) they would still get sent directly to the content all the veteran characters were at.
Thankfully, things look like they'll be very different in Legion.
Now, the presence of Demon Hunters is, to be fair, kind of undercut by the fact that we'll be able to create those characters before the expansion drops. The starting experience, which could take an hour or two, will all be available in the pre-patch for those who have already ordered the game (I haven't yet, but my policy is that I don't pre-order unless there's a release date. Barring some catastrophe that renders me unable to play WoW, I'm certain that I will get it - I honestly think this looks like it could be the game's best expansion yet.)
However, Demon Hunters and all the existing 11 classes will split off to do their own class things, establishing their Order Hall. These will then furthermore be split into three (two for Demon Hunters, four for Druids) in order to acquire the various artifact weapons.
While you'll be able to collect the artifacts for all of your class' specs, people will likely be doing them in different orders (I'm getting my Truthguard first, then Ashbringer,) and might save some of them for later on (I'll get the Silver Hand eventually, just to see the quests, but I don't know if I'll do that immediately or go off to explore the Broken Isles a bit first.) These quests will apparently take 20-40 minutes each, and they all seem to be solo scenarios, meaning that you'll have players getting to the Broken Isles 20-160 minutes after they start playing.
And then, of course, from there, players will be able to choose the order in which they quest through four of the Broken Isles zones - Val'sharah, Azsuna, Highmountain, and Stormheim. I don't know whether Suramar is going to be 108-110 or just a level cap zone, but by the time people get there, I'm sure that the playerbase will be fairly staggered.
I don't mean to suggest that the launch is guaranteed to be the smoothest ever or anything - there are a lot of complicated systems that will be at play, and I'm sure that, for example, instance servers are going to be working overtime to generate all the solo-scenarios for the millions of people who will be grabbing their artifacts.
There's also the sad fact that some internet trolls have basic hacking skills. Warlords' launch was certainly affected by a larger number of players than they expected to have, but the servers were also being bombarded with DDOS attacks. While Blizzard might anticipate a much larger number of players this time around, I guarantee you that the people sending those attacks are going to do it again this time.
Still, I think that the way they're structuring the expansion's leveling game looks fantastic. I suspect it will feel a lot more like an adventure, allowing you to explore and feel like you have more agency in your campaign against the Legion.