What factors affect the detail and clarity of a photo taken by a digital camera?

• Enough light to give contrast to the different details of your subject. If the scene is too dark, it doesn't matter how sharp your lens is or how many pixels your camera has. There won't be enough contrast to differentiate details in an image.
• The stability of the subject during the time of the exposure
• The stability of the camera/lens during the time of the exposure
• The focus distance of the lens is set to the proper distance for the intended subject
• The optical quality of the lens. In terms of acuity, this is usually measured in line pairs per mm that can be distinguished in the image the lens projects onto the sensor.
• If the lens or camera uses image stabilization, the effect of the position of the IS mechanisms on the image quality.
• The presence or lack thereof of an anti-aliasing low pass filter that slightly blurs the image to prevent color moire. It is in the filter stack in front of the sensor. Other filters in this stack include UV and IR.
• The pixel pitch of the sensor. This is usually expressed as the spacing between each pixel in microns (one micron = one millionth of one meter).
• The efficiency of the sensor. That is, how much of the light that strikes the sensor actually is detected by one of the photosites (pixel wells)?
• The amount of noise created by the camera's circuitry between the analog sensels and the digitization of the information collected by each photosite. The source of this noise can be dark current, heat related, etc.
• The demosaicing algorithms used to derive a tricolor value for each sensel which measures a single monochromatic luminance value for all of the light that was measured by each sensel.
• The effect of any noise reduction applied to ameliorate the effects of noise in the image. Since ultimately the amount of perceived noise is a result of the signal-to-noise (S/N) ratio, the more signal(light) that is captured without clipping the highlights, the less noise reduction will need to be applied and the more detail should be observable in the image.
• The effect of sharpening algorithms used when converting the raw data from the sensor to a viewable image, or applied after the fact to an existing image. Too much sharpening can create 'false details' or other artifacts in an image.
• Any other post-processing operations that 'shift' pixels. Lens distortion correction, for instance, stretches the results of the straight rows of the sensor's grid in curves that require the processing engine to interpolate pixels values. Some detail will be lost in such an operation.
• The pixel pitch of the device used to view the image
• The display size of the viewed image
• The distance from which the image is viewed
• The visual acuity of the person viewing the image