Neil Gaiman, the only writer besides Steven Moffat who we know will definately be writing for series 6, has revealed some of the difficulties of writing for doctor Who on his blog.
"The word came down from the powers behind Doctor Who that I was going to have to do another draft. The wonderful thing about books and comics is your budget is infinite: it costs the same to draw one thing as another, to write one thing as another. That’s not true of television, where a budget might stretch to prosthetics or CGI, but not both (quick! pick one! now rewrite to make it work) or you might find yourself being told that, no, we can’t make the Undersea Dancing Mermaids sequence work because the underwater cameras would blow the budget even before you factor in the mermaid tails but we already have a bird costume so could you just make all the mermaids into birds instead? And you’re sure you can, you can’t just quite figure out how...*
"Truthfully, I’m not grumbling. I’m loving writing Doctor Who, and so far, each draft really has been better than the one before. But we’re now at the point where people in Cardiff are nervously drumming their desktops and checking their watches. So I’m putting blogging again off for a few more days. Even last night, I went out with Amanda (she was making a guest appearance at a Steel Train gig), but spent most of my time backstage writing in a small green notebook, figuring out solutions. The Seventh Draft of My Doctor Who Episode should be done in the next few days.
You can read his full blog entry right here