Good lighting is essential! I don't depend on North light because where I live it is not always dependable. I also like to work in the early mornings and late evenings so I have really good color balanced flourescent ceiling lights. I like flouresent because it does not cause shadows. Day or night my light is the same.
A dedicated "studio" space....where you can walk in, pick up your brush and palette and get started. Having to shuffle functions within a given space is extremely difficult and a real deterrent to painting (i.e., clearing your paints off the dining room table so your family can eat...)
A solid easel that will grip your canvasses securely and is easy to adjust. Here's mine:
http://www.hugheseasels.com/
Directly behind my easel is a dark wall (covered with an old oriental carpet) because I cannot look into light. All of my studio walls are a neutral dark in color. I cannot paint in a room with white walls as it really messes me up...
Wall space so that you can hang your canvasses to dry. Placing them on a horizontal surface or a floor will attract dust to the wet surface.
Some kind of storage space based on the materials you use. (i.e., I have a lot of bookshelves in my studio for books and props.)