Fixing Image Quality
Quite a few things fall under this heading. The first being brightening/darkening.
Now, to a computer, there is a lot more information in the average picture (just jpg, not even talking RAW here) than is readily apparent to the eye. Different methods (levels, contrast) have different effects on the values assigned for the pixels colour.
Pictures can frequently be brought back to 'normal' - even if it starts by looking pretty much all black/white to start with. Extremes at either end are bad - dark pictures have 'noise' and overly bright can wash out all detail. Worth noting that I've had more success with brightening a dark picture than vice versa.
Noise. There are a lot of plugins and stand alone programs on the market to deal with the Skittle effect (taste the rainbow!). More often than not, they cost more than they are worth, since the free ones do an equally nice job. Sometimes you can get away with fixing the picture all at once, and other times, it's bad enough (will negatively impact the final image's overall colour) that it's better to apply selectively (with masks).
Chromatic aberration. If you haven't had personal experience with this, count yourself lucky. The problem with all these 'prosumers' having fancy-pants cameras is that unless you get the very expensive lenses (+$1k) you get that nasty halo around images. Particularly on the wide end of the lens and near the edges of the shot. Now Photoshop has a little thing called lens correction (which also fixes pincushion and barrel distortion) which attempts to tackle CA.
Problem is, that CA isn't a by-the-numbers type of problem. Well it is. Just very long and complicated numbers based on the lens, focal length and other controllable factors (which the camera companies do account for in their RAW correcting software), but a lot of unmeasurable factors (for all intents and purposes) like angle of light, brightness and so on.
What usually happens, at least for me, as that I get 'half-way' with the CA fix section, and get it the rest of the way the same way as I get rid of noise. Even then, a large image with a lot of people/objects can take several hours to tame the worst of it (and triply so if they didn't shoot in RAW mode).