This set of decals made by Design Police and posted up at Smashing Magazine should give you an easy way to point out the common mistakes people make in web design. From illegible text to unforgivable font choices, there should be a tag here for most of the things you want to say when someone shows you an iffy mockup.
If you print what Design Police calls their ‘visual enforcement kit’ on sticker paper and cut the tags out you can even stick em right on the monitor. I love the detail here, the siren with the marker in it is my favorite. Unfortunately, Design Police’s site has been taken down, so all we have is this low-res version. If anyone has seen a higher resolution version, let us know, we’d feel extra bad criticizing a design with a poorly printed tag!
H/t to Marco Tabini