Other people just get sick of load and rendering delays and use Wannabe or equivalent. If you must use images for navigation, be sure to provide alt tags or text alternatives. More obviously, it is always important to check to see that your site renders well in the various versions of Netscape, Explorer, Opera, iCab - as annoying formatting errors can, well, annoy. People have also been known to resize their windows to sizes other than the default, so keep those pages liquid, folks. CSS pages have been known to render a bit differently depending on the browser.
Speed is paramount.
Again, remember that everyone else isn't using your souped-up design workstation. It might be worth it to keep an old 386 or Mac II hooked up to the network as a reality check. You might be surprised how much you can speed up your site by recoding some key pages 'by hand', eliminating redundant attributes and tags. If you're really dedicated, you can go through and delete all the return characters and tabs for a negligible performance increase. Provide height and width attributes for all images, so users can see the page without waiting for all the images.