Joy and Julie's Seven Deadly Sins of the Web (revised):

1. Graphics which do not politely identify themselves with text.
2. Graphics which *must* be loaded in order to find out what the page content is.
3. Graphics which, when loaded, prove not to have been worth the trouble.
4. Graphics that move, shake, flash or are otherwise irritating.
5. A first-level page with no real content.
6. Web pages which require a particular browser simply to access.
7. Indifferent grammar and prolific typos!

Visit Web Pages That Suck where the above faults (and more!) are examined and explained in detail. I'd have tried to illustrate, but Vincent Flanders has done it so much better than I could, and he did it first.