They will be using the program, and they should be able to do it their way. Modes have their place, but not nearly as often as they are encountered. Another important item is to have visual cues to remind the user of how to accomplish tasks. This relieves their short-term memory, and lets them focus on the job at hand, rather than having to memorize procedures. In addition, you should assist the user's long-term memory by providing lists and menus with items that they can recognize.
Consistency is another very important facet of interface design.
If something works a certain way, some other similar component should work similarly. Why does it specify certain things in the way it does, and why have other things been changed. Basically, Windows is CUA '89, whereas the Work Place Shell is CUA '91, and later. It turns out that a whole study of the effect of colors on people had been done, and various hues and shades had been linked to certain psychological factors and reactions. Different strokes for different folks, I guess.