[Update #2]: The double experience event has been extended all the way until the Shadowlands pre-patch!
[Update]: Blizzard is also adding 'Legacy Loot' for Legion raids and dungeons. In other words, you'll now have an easier time getting some fancy new cosmetics for your alts!
If you ever wanted to introduce some friends to World of Warcraft's end-game, or simply level an army of alt characters to 120, this would be the ideal time. In order to encourage everyone to stay inside, Blizzard has now added a 100% experience buff for all players.
In reality, that bonus is even higher as this new experience buffs will stack with Heirlooms. Combine that with the currently ongoing Timewalking Dungeon event, and you can essentially speedrun your way to level 120, all the while earning new cosmetics and mounts from Timewarped Badges and the dungeons themselves. A pretty good deal, if you ask me!
The only thing you need to keep an eye out for is the time limit. The 100% experience buff will be active only until April 20th, so do make sure to start your leveling process as soon as possible in order to get the most use out of it. I learned that one the hard way with the recent Alterac Valley event as I 'had' to play it so much in the last two days that I now don't want to see its snowy mountains for at least another year.
On a slightly unrelated note, it's also worth mentioning that Blizzard has made it significantly easier to get Rank 3 Essences on alts. So once you do manage to get a brand new character to level 120, you should have a much more pleasant time getting them geared up and ready for raiding.
Have fun with the new leveling speed, and if you're looking to learn more about World of Warcraft, you should head on over to the official website.