How much editing of a photo is too much? A photograph's histogram provides a clue because it represents data on all the different tones and colors in the photo, from pure black on the left to pure white on the right and everything in between. There are lots of different opinions about histograms, but everyone agrees it's a useful tool because it makes it easier to see your exposure and if you have any pure blacks or pure whites in your photo. And almost everyone also agrees that it's undesirable to have too much (if any) pure white or pure black in a photo. In terms of the histogram this is known as "clipping" or "climbing the wall." If you want to know more about histograms you can check here at B&H or here at Digital Photography School.
The barn photo below is pretty much straight out of the camera with the exception of some cropping. There's no pure white or pure black in it anywhere--or anything that even comes close to it. That's one reason the photo doesn't seem to have much "pop."
The image below is the same photo edited to add more contrast and clarity, and to make the whites "whiter" and the blacks "blacker." I know there are some pure whites and pure blacks in it because the histogram shows data touching the histogram wall on both sides. To me this photo looks markedly different from the photo at the top of this post, which was also edited but with a much lighter touch.
When I first started editing digital photos in Lightroom, Photoshop, or any other photo processing software I could get my hands on I made the mistake that many photographers seem to make early on--I pushed my editing too far. I've learned to dial it back as I've grown, but I still like having a very small amount of pure white and pure black in my photos. Even that is farther than a lot of knowledgeable photographers would say is appropriate. That's where the "I'll color wherever I damn well please" side of me starts to kick in, but for the duration of my coloring inside the lines project I'll stay off those histogram walls.