It's well known that we humans can only take in a limited amount of information at a time, but when we take photos we often find ourselves trying to cram in as much as possible. This leaves us with photos with no single point of focus, that lack impact. In most cases, the power of a photo is inversely proportional to how many different elements it has. A close crop on a quarterback is much more powerful than a wide angle shot of the full field of players, and an isolated tree is more compelling than a busy forest view.
This is a lesson that I myself have learned over many years of making images, and these days my photographic 'style' is one of simplicity. The best images have an emotional impact, but if the viewer has to spend time constantly scanning various elements that impact is lessened.
The first way to simplify an image is to simply get closer to your subject. In most cases you don't need to include the entire subject in the frame - the brain can figure out for example that the weathered door is part of a building, so get in close to the door and perhaps include some of the surrounding wall in your image. Or to take it to another level, maybe the door handle is the most interesting part of the image, so why not let that fill the frame (this time including a little of the door)? What you are effectively doing in cases such as this taking advantage of how the brain works - our "working memory" scans the image but the incomplete image causes us to draw on our experiences to fill in the gaps. It's the process of filling in those gaps that results in a more engaged viewer.
A second way to increase the impact of an image is through careful use of color. Images with simpler color palettes are often more effective. Think of the forest image in fall with a red tree amongst yellow ones - this will be more powerful than an image with a rainbow of fall color (or no color variety at all). Remember in this case though that we're attracted to bright things first before dull things, so if you have a particular point of interest in the photo you will need to keep that in mind.
There are other more complex techniques available such as selective focus, contrast management etc. These more technical approaches require more skill, and should be viewed as secondary techniques to the key ones outlined above. What they all have in common however is that they are all strategies to 'persuade' the viewer to look at a photo in a certain way to increase its impact.
In the end, all you need to do is remember to keep it simple. Your photos will be better as a result.