I mentioned in a previous post that I'm not a big fan of "over processing". Since this term is way too general, I wanted to explain what "over processing" means to me and also talk about a few exceptions and qualifiers.Browsing the internet, I can see that the majority of photographers agree that an over processed picture is one that does not look really natural (the Photoshop look). Examples: over saturated colors that are not seen in nature, extreme sharpening, skin tone and softness that fits a porcelain figurine more than human being.
I have to admit, the line between "just enough" and going over board is sometimes hard to find. For me personally, that's one of my weak points. For a long time I opted for very little or no processing because every time I tried to push it I fell into the over processing trap. I like to think I am getting better at it, but I'm nowhere near where I want to be.
Now, a major exception is if you are aiming for something closer to an illustration, or more accurately, a hybrid between a photo and an illustration. Some of these could be really interesting. Another case could be if you are going for a certain vintage look, like the Technicolor movies of the 40's and 50's. (I personally like those occasionally. Think "The Wizard of OZ").
In general, less-is-more applies here. But rules are meant to be broken (once you learn them and know what you're doing).