Nail, Bang, Head
I feel that all to often those of us who spend every day working, surfing, reading and thinking about the Web lose sight of the fact that it’s still a new medium and there are lots of folks out there who don’t spend every day working, surfing, reading and thinking about the Web. This could be newbies, your clients and stakeholders or any number of people who might have a vested interest in understanding what makes the Web tick.
This is something that can be extremely difficult to remember; by default, anyone involved in web development is themselves an expert web user. Watching less experienced people using the web can be incredibly enlightening (especially the things that look like real howlers, like the first time you see someone typing a URL in to a search engine because they haven’t figured out the difference between that and the browser address bar).
I maintain a strong focus on usability in all of my web projects, but I almost always consider usability from the point of view of an expert user. Figuring out how to deal with people who have very little experience of the web can be a lot harder.