I was very lucky as I was allowed to pick the supervisor and the topic of my choice. As next year I will be a masters student, it meant that this year my dissertation was very short and only 3,000 words. I had so much more I could have written, looking back now. Cutting the word count down was particularly challenging as everything seemed just too important to cut out.
However, I think the main difficulty is in the preparation of the viva.
I've read, reread and triple-read my dissertation, reread the references and then on top of that tried to think of the questions which they could ask, both the obscure and the obvious- I always think the questions with the most obvious answers are sometimes the most difficult to answer.
Fingers crossed it goes well tomorrow!