The beta is purportedly a much more complete version of Legion than what was seen in the alpha, and as such Blizzard wants all testers, new and old, to start over. Those invited won’t be able to import their characters or create new ones but instead will have to choose from several level 100 templates. Blizzard said that there’s no NDA during this beta period, so chatter about it to your heart’s content.
Even if you haven’t gotten a beta invite yet (or don’t want one), you might be interested in reading through the expansion’s beta patch notes. Legion introduces a new hero class, the Demon Hunter, as well as five additional zones, 10 dungeons, two raids, the artifact system, class halls, a revamp to the transmog system, and world scaling and questing.