1. Lack of Contrast
Designing with colors that blend into the environment and/or don't have a lot of color. Always design signs with an element of color in order for them to stand out in their environment. If your building color is black, use a lighter color to create contrast.
Most business can make a fatal mistake by placing their signs too high or covered by another object or even next to something that may be distracting. Make sure it is eye level and visible so that your message is easily seen and remembered.
Believe it or not businesses like to cut corners with building signs and it's just not worth it. After all, you're representing your brand. Get the most durable and attractive sign that you can that can prevent fade and stand up against harsh weather. Remember: That will be first thing your customers see and the first thing they will remember when thinking about you for their next purchase.
4. Too Much Going On
Keep it simple. You want to be able to attract customers' attention within the first two seconds and convey your message. People are visual and including a lot of text or various images is going to divert their eyes elsewhere. Sometimes the opposite is possible too, having a design element that may not be enough information or confusing to the unknown eye.
When in doubt a larger sign is always going to attract more attention (especially when positioned outside the business). However, make sure it is not too big that it obstructs visibility.