I'd be curious if you have identified distinguishes a truly exceptional product photograph from a just alright one?
I'll bite :)
A - Technical Perfection: Properly lit, picture in focus, correct depth of field, correct color balance, framed appropriately. Product is correctly staged for the best appearance and to highlight specific features. Background and any other elements in the picture are chosen purposely and help with the narrative.
B - Clarity on Purpose/what you are selling besides the product. All of the above can be manipulated to tell the story you would like. Going back to the old adage "A picture is worth 1000 words" You have a 3-5 second window (at the most) to convey everything you want in a product shot or to entice the person to slow down and examine it further.
You need to be clear on your "why" before you start shooting. Every choice and technical/creative decision must support that goal.
Is your sole purpose to convey information, like an ebay listing? Okay, so you need to have the product take up most of the frame, be in crystal clear focus, with a filter or lighting that approximates the natural light the product will be viewed in. You want to choose a background that doesn't conflict with your item, if you use any colors you want to know color psychology to make sure that what you are using is sending the correct signals to the viewer's brain. How is the product best displayed? If it is clothing should it be on a mannequin or on a model? Do people need specific information in the picture like labels? Your product will be viewed as a thumbnail image, how do you make yours stand out? Put a border around it? Use a different colored background? Use text?
For larger products, what problem are you trying to solve and what story are you telling? Are you going to just feature the product itself? Will you show it in action? Do people really care about the product itself, or what it can do for them? What emotions or feelings are you trying to bring out? Is this a stand alone picture or part of a series? Is your purpose only to show the product once or be used longer in marketing? How do the pictures fit in with your overall brand? How will the pictures be used (example: website, print, blown up for banners) and what technical detail do you need?
All of the above questions in the ebay example matter, just more so, as there is more at stake and you want to create cohesive pictures that will last you a while and fill a variety of needs.