The most critical misconception is:
Good graphic designs on the web should be made to look just like good graphic designs on paper
This mistake gets made all the time, especially as a lot of print-trained designers move into web design. This is in some sense a corollary of the principle that web design equals make it usable and intuitive, not make it pretty to look at. In fact, the best designs are a synthesis of these, but the usability should always come first: the aesthetics will grow out of it naturally.
Here are the problems with emulating print design:
- With paper the designer has control of the size, the layout and the proportions of the design. Different browsers, screen widths, color depths, font distributions, etc., mean that you have less control than you think over what the user finally sees.
- Most computer screens have a resolution of about 70 dots-per-inch (dpi), and some people still browse in 8-bit color. Paper designs can be of arbitrary color depth, with resolution so good that dpi is almost meaningless. (Hi-res screens are in the works but promising technologies like LEPs are years away from the consumer market). You cannot hope to accomplish the same things with text, color, shade and gradation on a web page that you do in print.
- Paper designs can be arbitrarily complex, but you can still view them instantly. Complex elements or animations in web designs can take many seconds to load, especially with a slow connection.
- Paper designs can't be interactive; web ones can and should, when interactivity is appropriate.
For the most part, people using the web are looking for information, entertainment or commerce, and the first duty of a web design is to enable this. Period. In particular, they aren't looking for flashy, glossy junk. The (arguably) most successful website in the world, Yahoo!, for the most part keeps it simple: text, links and some pictures. The same with Amazon. And Google. There's a reason these sites have done so well: people find them useful and easy to use.
Note: If you find this sort of thing interesting, there is a wonderful but not-well-known introduction to basic web design principles called the Web Style Guide, at http://www.webstyleguide.com. It is the most concise, direct and convincing treatment I've seen on the subject.