I love having a design wall in my sewing area. I find it really helps me as I plan a quilt. If you have the wall space, definitely add one. The bigger the better. Mine is 8 feet square, and I sometimes even wish it was a little bigger than that.
I also find a large cutting surface to be very helpful. Most tables are too low for me, and I end up with an aching back. My cutting table is a hollow core door placed on top of a day bed from Ikea -- it happens to be the perfect height for me. And since my sewing area doubles as a guest room on occasion, it's a good use of space for the dual purposes of the room.
What else -- lighting. REally good light can make your space so much more enjoyable to work in. Definitely an area I need to improve in mine.
RST