When designing for any new project, the work itself is the most obvious consideration. Artistic styles vary so widely, and hence, the style of framing should go hand in hand. However, what I tend to find equally, if not more important is the art material itself. Paper type is a good example, glossy photos for instance generally look best when the edges are held flat with an over-mat, the process where we cut a window in a mat board to show a specific area of the art.
On the other hand, handmade print works usually look best float mounted on top of a solid cotton mat board, to show the edges and allow for some gentle waviness left over from the actual printing process. Size and proportion played a massive role in a successful design. Over-matting vs. float mounting requires a completely different take on scale and spacing.
Finally the shape, texture, and color of a picture moulding will convey so much of the final mood of any given project. We love it when its possible to accent some otherwise unnoticed element of the original work, be it photo, etching, screen print, or keepsake item!