TOO MANY TYPEFACES
Using too many typefaces in a logo, website, etc. can make a design appear busy, non-cohesive, and unprofessional. Play it safe abd stick to the KISS principle on this one; settle on 2-3.
LACK OF HIERARCHY
Hierarchy determines which items you see first in an image. Designers pay attention to hierarchy and use it to their advantage by varying the size of their fonts (titles and headers should always be more prominet than body text), using color wisely.
Contrast also helps with hierarchy. A lack of contrast can alter legibility and make an image appear washed out and undefined. A mix of both light and dark colors creates balance.
DIZZY COLOR PAIRINGS
When objects with similar color values are placed near one another, it can sometimes have a dizzying, vibrating effects. Use contrast and be cautions of color pairings, especially when you are layering colors on top of each other.
LACK OF NEGATIVE SPACE
The negative space around an object is often just as important as the object itself; it provides a cushion and a place for your eye to rest. Avoid cramming objects and text into a design and be aware of the entire composition, not just the main components.
CENTERED PARAGRAPH TEXT
Large amounts of text should never be center aligned. It alters legibility, making text harder and more frustrating to read.
NO GRID SYSTEM
Haphazardly placing objects on a page without rhyme or reason is never a good idea; it has the potential to make a design seem random and unintentional. Use a grid system and align objects to create order.