Purely FWIW, regardless any new & improved features, plug-ins, & other assorted gimmicks, this is what I find I repeatedly use over the years regardless the image editing app -- *Maybe* it'll help someone when they're looking at image editing software or trying to do more than a very quick fix?
TO me the most important adjustment is always levels, & that's the 1st thing I look for when I fire up an image editing app I haven't used before -- a surprising number of editing apps don't have what used to be a standard requirement, perhaps because they have some automatic version they hype in their product advertising? I think it is, after all the basis for most automagic fix-it functions or features. And full auto's fine for batch processing -- it's just that you have eyes, the software doesn't, so you can do better than any algorithm. Usually to set levels you look at a Histogram, where you have 3 sliders -- basically the one on the right is for white, the one on the left is for black, and the one in the middle controls midrange or gamma. Looking at each of the 3 [RGB] colors individually, you make the lightest colors in the image closer to white, & the darkest colors closer to black, increasing the dynamic range of the photo. Different apps expand on the process, maybe adding eyedropper tools to sample the brightest & darkest parts of your image & so on. Whether you're working with scanned prints or negatives, or something you got from a digital camera, the electrical sensors that captured the image data can't do a perfect job of it, so the results are processed by whatever built-in electronics before the image is written as a file, & then your software may have a go at it... Most always you can improve on those results by adjusting levels. Here's a couple of articles on Histograms -- http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml -- http://www.cambridgeincolour.com/tutorials/histograms1.htm
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One tool I find myself using often, especially if I'm working with something I scanned, is the clone tool. I think most people are familiar with, have used clone tools in various editing apps, so I won't bore you with the basics -- here's what I look for, but 1st, why I mention it. Our brains process what our eyes see, & a key part of that process, buried very deeply, is pattern recognition. This part of our brains is also tuned to watch for movement & look for contrast. If we spot something out of place our eyes are drawn to it, creating a sense of movement that reenforces our sense that something's wrong. The reason our brains work this way is to spot threats, e.g. a lion in the bush that wants to eat you, so it can understandably cause us to feel tense. An artist &/or designer might use that on purpose, but if you're repairing something that's the very last thing you want -- you or the people viewing your image may not see anything wrong, but for some [maybe subconscious] reason no one likes it.
When using a clone tool what I try to do is avoid ANY pattern & contrast, & I do that by having the tool set as an airbrush with feathering & partial transparency, using one-click shots, changing the source or sample target often, rather than any dragging or painting motions. With some of the editing apps I've tried, using the clone tool as an airbrush that way wasn't/isn't possible.
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Especially since digital cameras became the norm, I often find layers Very hard to do without. It's not so much that digital cameras are so much worse than their film-based ancestors, but we don't have the benefit of someone else processing our film, using processes &/or methods based on decades of experience using tools we could never afford. Outdoors, cameras have to deal with difficult lighting -- indoors, they make do with a very often inadequate flash... either way there are often parts of the picture that should be lighter or darker, but it wouldn't work to adjust the entire image itself. Using a layer mask, often a gradient, you can lighten or darken portions to your heart's content. Sure you can use a brush to lighten/darken, but to mimic a fine gradient you'd have to have the brush feathering set a bit high & be very precise in your "painting", or make a lot of individual, feathered selections. A gradient layer mask makes the whole thing seem trivial.
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I also like the capability to paint masks or selections -- almost to the point of demanding it. There are all sorts of selection tools, and even apps & plug-ins that focus just on making selections so you can drop out &/or change backgrounds etc., but I've found what works best for me is working backwards, getting rid of everything else rather than defining just what I want amid a complex background. I'll start with very rough selections, often using the rectangle selection tool, working my way from the surrounding area in towards whatever it is I want to isolate, then painting the final step around the border. I may still use one of the other selection tools after that, especially if I'm masking something like grass or hair, & I've found working this way they seem to work better than if I started with them in the 1st place, e.g. using the magic wand tool there aren't any stray pixels I might miss caught up in the selection.