Mike Rundle has written an article about site design and the C.R.A.P.. There are various examples of what C.R.A.P. means on the web (Robin Williams first coined the acronym), but for me it’s this:
Elements that aren’t the same should be very different so they stand out, making them “slightly different” confuses the user into seeing a relation that doesn’t exist. Strong contrast between page elements allows the user’s eye to flow from one to another down the page instead of creating a sea of similarity that’s boring and not communicative.
Repeat styles down the page for a cohesive feel — if you style related elements the same way in one area, continue that trend for other areas for consistency.
Everything on the page needs to be visually connected to something else, nothing should be out of place or distinct from all other design elements.
Proximity creates related meaning: elements that are related should be grouped together, whereas separate design elements should have enough space in between to communicate they are different.