I am in the Beta for World of Warcraft: Legion. I downloaded the Beta client and installed it. Then I got down to business and made some characters so I could get on with testing. Immediately I hit a big change.
Your character's abilities are going to be significantly, if not radically, different.
No matter what your character's class or specialization is, you will find that it is entirely different from what it is on Live. I recommend that you spent some time on a target dummy to get familiar with your character's new suite of abilities. You will have to relearn how to play your character. Count on it.
So far, I've gotten familiar with how the new Fury Warrior plays (as that is my main character's Class and Primary Specialization) and I've played part of the Demon Hunter starting zone. Due to known phasing issues in the Broken Isles World Server, I've avoided actually going to the Broken Isles. Once that's resolved, I'll go ahead and do that; until then, I'll just finish the Demon Hunter play experience and get familiar with other classes and specializations.
When I do get into playing (and testing) Legion content, I'll start on my Fury Warrior and do Stormheim first. I intend to take different leveling paths on different characters, and I intend to test the initial max-level world content.