There is software compatibility and user compatibility to consider. Ideally there should be special attention given to figuring out what functions are to be represented by what icons within the GUI. This kind of universal understanding is often overlooked and confusion and frustration are the result of poorly thought-out interfaces. For icons to properly represent a function they need to be intuitive. Let's look at a simple task and the applications used for simply getting a photo and sending it in an email.
Imagine you have a CD containing photos from your last vacation stored on it and you want to send some of those photos to your friend. You’ve never used a Macintosh before to do it, so you’re not quite sure what to expect.
There are potentially two issues here that Mac OS X handles extremely gracefully, without requiring any special knowledge of specific applications or artistic skill and training. You proceed to insert the CD ROM in just the same way as you would on your PC and immediately an icon appears on your desktop in the image of a small CD. Ahh, okay -- that certainly looks like the object you just put in the computer so you continue onward. You double click the CD icon and you see a preview of the photos stored on the disc then drag the one you want onto your desktop (remember that you know nothing of iPhoto yet).
Now you want to email the photo to your friend so the next task is to figure out what to click to open your email program. How about that little postage stamp icon sitting in the dock? It makes perfect sense to me that before you can send anything through conventional mail you’d first need to put a stamp on it and email is a lot like writing a letter isn’t it?
You click it and you drag the photo into the body of your email. You add the email address of the person to receive it and voila! We have now successfully copied a photo from your CD ROM, and emailed it using rather intuitively thought out icons and simple procedure. No special knowledge was needed as the tasks to be performed were answered by using universally understood graphic representations suggesting which actions to take.