Rule #x - Test your design on lower resolution/quality screens. I see a lot of articles/blogs/whatever that look like crap on my Dell 1680x1050 (non-ips) work display.
And on different OSes. For example, many designers work on Macs and don't bother testing on Windows, then get surprised when users complain that the type on their website is unreadable -- OS X's rasterization makes light fonts look much bolder than Cleartype on Windows.
