A little while back I wrote Buying a Camera[a], and as a follow-up, I'd like to discuss the concept of image quality. Image Quality sounds like it should have something to do with the quality of the resulting photograph, but it rarely does.
When you take a picture, you start with a scene that you have created, found or traveled to on purpose. You move the camera around for composition and perhaps wait for the lighting to be right. Then you select perspective (focal length), sensitivity, white balance, shutter, aperture and focus. You select the time when you press the shutter button. The sensor registers something, and this information is amplified, read out and processed into some kind of image file. That file is then post-processed and maybe printed according to a certain process on a certain material or displayed on a monitor with certain display parameters. The print or display device is placed in a certain place, maybe in a book (print) or hung on a wall (print or LCD frame), and lit in a certain way. Finally, an observer, with their own particular likes and dislikes, views it. If we're lucky, we'll have managed to communicate our vision of what we thought was interesting when we stood at that scene so long ago, and the observer will like the photo. That is the quality that matters. But we don't call it "image quality".
When talking about image quality, we specifically refer to technical image quality. That is, the amount of noise, overall sharpness, dynamic range and amount of distortion and aberration. A high-IQ system will have low noise, sharp images, wide dynamic range, low distortion and low aberration. It is important to understand three things about this:
It's just one link the the chain: Of all the links in the chain from scene to observer reaction, we are really only concerning ourselves with the part "The sensor registers something, and this information is amplified, read out and processed into some kind of image file". As you can see, there's a lot more to it than that.
The IQ must be measured in the proper situation: The Noct Nikkor[b] lens, for example, excels at night time photos of stars, but due to its design it is significantly worse than any other lens in daylight[d]. If you measure the IQ in the wrong context, you'll end up with a system that performs much below what you expect or paid for.
Photography, as an art form, isn't about making a scientifically accurate replica of the world, it is about creating something aesthetically pleasing: We use limited focus, grainy noise and compressed dynamic range all the time to create photos that are interesting and beautiful instead of boring. I can process a portrait in such a way as to make every zit and wrinkle clearly visible, but who is interested in showing that to their friends? If you are doing medical imaging or research, then your goals are obviously different - but if you're reading this, you probably aren't.
All that said, having an image capture system with high image quality means that you minimize the information loss, and that is something good. How good it is, and how much you are willing to pay for it, is something that you need to evaluate yourself - I recommend using the method I describe in Buying a Camera[e]:
How many times have you failed to capture the image you wanted due to low IQ? If the answer is "rarely" or "never", then ignore IQ.
Is it low IQ in the camera, or low IQ in the photographer? If you suspect a lack of IQ in the latter, focus your efforts there and not on the camera.