I played Diablo 3 at launch; time zones didn't matter, I couldn't even get logged in.
Problems are always going to happen when you have millions of people trying to connect at once. I imagine the problems will be amplified as they're all trying to download a mandatory update.
I wouldn't really say the amount of people buying the Xbox One (and downloading the update at once, and same with PS4) will be millions.
Maybe 2.5 Million at best. With probably half of that downloading the update at Midnight (for their TimeZone). For example, in America, Eastern will go first. Probably a couple Hundred Thousand. They should be done in 30 minutes top. Most of them will be done by the end of the hour, and what time Central will go, repeat for Mountain and Pacific.
I don't have any problems downloading 50 MB updates for Call of Duty at midnight, and there ARE millions at that moment.
Good thing everyone in the world is downloading on a set schedule.
Come on man.
Its not a set schedule, it's just that's how it works. Everyone is an hour ahead, they just have to deal with the Hundred or so thousand in their area, instead of dealing with millions all at once. Some will be done at Midnight, and then the rest of the day will be random, but should still be maintainable.