For starters, know that first-time visitors notice the time it takes to load the first page. Nothing drives your audience away faster than making them wait around for fat graphics files to show up. The "eight-second rule" isn't a bad idea for a benchmark. Anything slower than that needs some attention. If you're not sure which graphics are the problem children, try using Web Site Garage's GIF Lube tool. The GIF Lube will check your site, compress your images and show you the faster-loading results.
Moving on to the realm of the really obvious, some simple double-checking can save your potential customers from rolling their eyes and saying "Oh, there's a typo" or, "Oh no, not another 404!" SiteOwner.com lets you take on both birds with one Web-based tool, the Spelling & Link Checker. Plug in your URL, and the tool gives back a list of possible misspelled words, as well as broken or missing links on your Web site.