Because of the focus on simplification, all icons should be drawn in the “on-the-shelf” perspective (see the Gnome HIG). Adding perspective would only increase the need for detail which is exactly what you are trying to avoid.
The icon theme consist of SVG artwork. Having all icons as vector art makes it possible for the user to set the size according to his ability to see with the artwork staying sharp and not introducing any scaling artefacts caused by interpolation. The theme spec enables you to have your original artwork at any size and define the scale in the index.theme file. I reccomend, however, you keep your icons at a nominal size of 48x48 points. I will be giving suggested stroke sizes so you can save yourself from doing sophisticated calculations ;).
As you can test for yourself, thin lines are totally invisible for visually impaired people (see Section , “Testing Your Artwork”). The minimum line stroke width for outer border should around 3.5pt (Some illustration packages don't allow to stroke a path from only one side. You can overcome this limitation by creating a copy of the object and stroke it with 7pt below the original object). It is possible to stroke inner object with a lighter line, but you shouldn't go below 2.5pt (5pt resp.).
There is one technical difficulty when using SVG icons as gtk+ stock art. gtk+ will always scale the artwork to the destination rectangle, not keeping the aspect ratio of the original. Your icons would look skewed unless you provide an empty bounding box around it. Safe precaution is to draw an unfilled rectangle with no stroke around the artboard (48x48pt).
The fact that you are only working with two colors turns quite difficult at times. It is good to keep a certain balance between black and white areas. Try to keep away from doing 'hollow' icons, the amount of black should be over 30% when possible. There are of course exceptions, such as a sheet of paper.