Have you ever seen a beautiful image but something didn't seem quite right about it? Maybe the flaw wasn't obvious but very present in the image even if you couldn't put your finger on it. Over the years, I've taken many such photos. Images I really liked but felt they fell short of the best that they could be.
Once I got serious about photography and began to study various techniques and tutorials from well established photographers, a whole new world opened up to me. One of the first tips I learned concerned keeping the horizon level in your images. Beach images are some of the best at demonstrating the importance of keeping a level horizon. The two images below are identical. However, the image on the left has a slanted horizon - slanting up from left to right. It's not glaringly obvious but it's there and detracts from the image, especially once you notice it. The second image has a level horizon which results in a more visual pleasing photograph. Once I learned this very important tip, it's often the first thing I look for in an image that has a horizon, whether it's one of my images or someone else's.
Slanted horizons can easily be corrected with editing software but it's always much preferred to get the shot right in the first place so the correction doesn't have to be made after the fact. And, thankfully, with a just little extra attention paid to your framing before you press the shutter, it is one of the easiest mistakes to avoid.
One last closing thought, it's always okay to break a rule of photography for artistic reasons. My advice is to make the rule breaking obvious so that it doesn't appear that your subjects are going to slide out of the picture!