2. You think you will work with inspiring writers. Well, you may. And on the other hand, you may not. Not all writers can teach, and there are some duffers out there. I've heard horror stories of tutors who were too busy with their careers to turn up or mark work, and tutors who were so disillusioned and cynical about writing, their mission seemed to be to deter students not encourage them. You may also be taught by unpublished PhD students.
3. You think you will learn all about writing craft. Think again. Most - if not all - MAs seem to work on the osmosis principle ie that if you read enough good stuff it will rub off on you. This is true, but it also helps to teach craft.
4. You want to write commercial fiction. The MAs are oriented towards literary writing, the sort that wins prizes like the Booker or Whitbread. You may find the staff a bit sniffy if you want to write commercial fiction, and you may feel annoyed at the plaudits being rained on some 22 year old whose writing is (in your opinion) completely up itself.
Having said that, I loved my year and would do it all again because...