Why is everyone freaking out? This was a possibility. If the stories and acting are good, that's all that should matter. Now, I do have questions. In the books, the 13th Doctor was a ginger. So this deviates from that.
I find that part unfortunate, but the show is canon and can change it as needed. As a fan of anything, you accept that part. Star Wars, Star Trek, Game of Thrones, etc... it just cones with the territory. Fans can retcon however they want.
I am worried this will kick off a 'The Doctor should represent [fill in the blank] minority in subsequent regenerations' bit of nonsense though. Doctor Who is the ultimate in flexibility for a show. They can go anywhere in space and time, the character can regenerate when he/she dies, the travel companion rotation is expected... Its basically the perfect recipe for a never ending show.
Now, they've made it even more flexible in how they cast. They can essentially cast the best actor now, regardless of anything else. That's got to be a relief for producers. They can really write to their strengths. And I hope that's how this is taken.
No agenda, now political commentary, nothing like that. Doctor Who should be above that. The goal should always be about writing great stories because it's ultimately entertainment, not using the platform as a vehicle to condescend to viewers.
That's really the only small worry I have. That people will attempt to turn the Doctor into a political lightning rod to shove ideology down people's throats, instead of just enjoying a great show.
I expect that Mrs. Whittaker and Mr. Chibnall will give us a fresh new take on the traditional Doctor Who story and a few surprises, and that's what I'm looking forward to.
Plus, lets be honest, she's hot. This will be the first Doctor I get turned on by, so that's got its own appeal.