Over-editing in Photoshop is a chronic problem. When photographers first get and learn to use Photoshop, they often are in awe of its capabilities but do not have the skills to use it properly. As a result, many start out playing with filters and plug-ins and over-use them. Sometimes photographers feel Photoshop is all powerful and take images that should have been in a reject pile, and they try to “save” them. As a rule, Photoshop should not be used to save unacceptable photos. If a photo is out of focus, blown out, severely under-exposed, or has really awkward composition, Photoshop will not make it drastically better. Used in excess, it can actually make the image worse.
via Over-Editing in Photoshop: How to Avoid 25 Common Editing Mistakes.