It really is a cliche, but the hardest part of any script writing task is staring down at a blank piece of paper and realising at last the task that lies ahead. I'm at that stage right now with "Frank's Apocalypse" - I have episode one totally plotted out, but it's still an impossible task to actually start the writing process.
The problem lies, I think, in knowing that up to that particular point everything is in your control; but when you start writing the script all sorts of things can happen. Ideas can change, or no longer look 'decent' on the written page. There's also interpretation; the second your ideas are written down, they're no longer just yours - they belong to a production team, and eventually, the audience that will view what it is that you've written. It's scary, because what happens if that same audience don't like what you've made?
I guess it's just me being a silly writer. I should just sit down and write the bloody thing; but when you take something as serious as I do it's hard to settle for anything but "perfect".