Others are not so obvious. And new ideas emerge every day, as do new technological advances. The differences between a web site and other media in the way that messages are conveyed are distinctive and important to realize. Just the look of the word or image on a page is different on a computer screen. You can control the look of the printed page, but you can't always control the look of that same information on a computer screen.
Different browsers show the same images differently.
The viewers' screen sizes differ, as do their skills in using a computer. Your site has to be reviewed by several different browsers to see and correct the differences. The clarity of the image is rarely as good as it is on the printed page. Allowances must be made for these differences in designing a site. Some type faces are more readable than others, for example.