According to usability guru (sure he is … come on) Jakob Nielsen, the worst mistake made in terms of Web design today has to do with fonts.
In his article, “Top Ten Web Design Mistakes in 2005,”
the readers of Nielsen’s newsletter put legibility problems at the top.
About two-thirds of the voters complained about small font sizes or
frozen font sizes; about one-third complained about low contrast.
Nielsen has written about this before; see his 2002 column, “Let Users Control Font Size.”
Runners-up on Nielsen’s list of Internet annoyances were non-standard links and the use of Flash. And the list goes on…