In some ways, Legion is the opposite of Warlords of Draenor. In Warlords, we didn't get any new toon incentive - no new class or race was added (though the new character models for the old races should not be ignored as a serious improvement to the game.) It also funneled absolutely everyone into the same area at the same time - while Alliance and Horde went to opposite ends of the continent for their first full quest zones, everyone was put through a pretty bottlenecked series of quests in Tanaan Jungle - the only glimpse we got of the zone before it got super fel-corrupted.
The launch of Warlords was far from perfect - DDoS attacks (to be fair, not Blizzard's fault) made the first few hours pretty hard to play, though after the first day things got better.
So what Blizzard is doing is kind of clever. At first, the unprecedented release of Demon Hunters before the expansion itself launched seemed like a bad move - in the past, we've had people starting their Monks, Worgen, Death Knights, etc. while others decided to steam on ahead on their mains and go to the new continent, nicely dividing the players into two or three groups (three when we have a new race for each faction.)
But actually, releasing Demon Hunters tomorrow actually plays into the clever staggering that the game will be using this go around.
For one thing, we got the systems patch weeks ago, meaning that people have had time to update add ons and get used to new rotations (and pick out their best transmog set.)
Now we'll be getting the Demon Hunters and the biggest pre-launch event we've had since Cataclysm. But then it will be three more weeks before the expansion itself launches.
Not every player who intends to play Legion will have a Demon Hunter yet, though. Those who want physical copies (including the physical collector's edition, though I think there's digital version as well) will need to wait until the 30th. So not every player is going to be starting their DHs the moment the servers come back up tomorrow (I'm assuming that's when they'll be unlocked, as opposed to midnight tonight.)
But then the expansion itself is staggered in a clever way as well.
Players will have a series of quests that eventually send them off to Dalaran in its new location above the Broken Isles. From there, the player base is divided not in two, by faction, but in twelve, by class. While some of these quests will still take place in Dalaran, a lot of them shuffle people off into separate zones and instances. And shortly after starting this chain, players will get split into thirty-six different quest lines as each player pursues their own artifact weapon. Yes, there's some overlap - I know that Affliction Warlocks, Unholy Death Knights, and Balance Druids all get sent to Deadwind Pass, but that's really not too bad, having to share a zone with a twelfth of other players.
And then, of course, you get into solo scenarios.
After completing the quest chain to get your artifact weapon, it will have already been enough time for different players to be finished at different times. And after that, each player is going to choose which zone to start in, meaning that even after all the class-and-spec specific content, you're still going to have people going to different parts of the Broken Isles (and probably dividing at least some of those players between different parts of the zone depending on faction.)
This is really a pretty complex and comprehensive way to get players to spread the hell out. Sure, Dalaran will probably be Dalalaggy as people arrive, but they're sent off pretty quickly.
Obviously, anything can go wrong, and simply having so many players logged in at the same time will probably put strain on the servers regardless of how spread out things are. But I think Blizzard is trying a new and very clever plan to make the launch-day experience far better than it has been in the past.
Looking forward to it, but I'm focused more on Demon Hunters tomorrow! And my laptop has a new SSHD, so I might actually be able to do more than simply pick out the hair/face/skin color/etc. of my Demon Hunters!