Although I generally focus on the functional aspects of web sites, visual design has an effect, either positive or negative, on site usability.

I'm surprised too...

Research by the Software Usability Research Laboratory (SURL) at Wichita State University [1] shows that a well-designed web site is perceived as being more functional than a poorly designed one, even where the actual functionality is identical.

For someone, such as myself, for whom function prevails over form, this is alarming. How dare mere users be distracted by the pretty colours instead of seeing the essential utility of my application.

But then again...consider the evolution of the Windows 'Start' button from Win95/NT through XP and Windows 7.

Windows NT

Windows XP

Windows 7

From a usability design perspective, the crucial difference is the tiny gap at the bottom and left in the original version - highlighted here in red . This gap means that to get to the start menu you typically sweep the mouse cursor to the bottom left corner then make a small, precise adjustment to target the button - quite annoying. In the later versions, this space has closed so now you just whack the mouse down-left as far as you like and there you are. The Windows 7 version looks like there's a gap at the left, but functionally there isn't.

What not to do

Don't let your designers have free reign. A frightening number of web sites of design firms (architects, graphic designers etc) are unusable horrors. Pretty, possibly award-winning, but not actually any use to an outsider (who might be a potential client!).

I have had a client decide on the layout of part of their web site before they have any content and ended up saying things like 'I want something dynamic is that section - I don't care what it is, use a dancing penguin'. This is actually true.

What to do about it

Any design has to be built and maintained. The more features you build in, the more things there are to break; if it doesn't achieve a functional goal, maybe you should leave it out (say you're 'deferring' it, which will makes the designers feel better).

Remember that your users are not you. They didn't build your web site (or program or whatever) and don't know how you expect it to work. You design needs to be aware of what people need to achieve, not just covering function points from a requirements specification.

When it doubt, keep things consistent. If your users have to re-learn where the controls (menus etc) are on each page they will quickly become frustrated.

Beware of things that look good the first time you see them but get in the way of regular users. If someone has to sit through your 30 second flash introduction every @#$% time they visit your web site, don't expect many repeat visitors

References

[1] Visual Appeal vs. Usability: Which One Influences User Perceptions of a Website More? Software Usability Research Laboratory, Usability News 112