Users can't tell what your company does instantly upon seeing the site and header.
The site has any kind of intro, welcom, or "splash" page - 25% of users leave a site immediately upon landing on an intro page.
The site uses any animation, auto-play video, scrolling or flashing text or marquee.
Use of pop-up windows.
Music or any other audio automatically playing.
Mystery Meat Navigation -- navigation elements designed for aesthetics rather than usability -- any navigation element or icon that does not have explicit and user-friendly text (text that appears only on rollover is still Mystery Meat Navigation).
Clashing colors or excessive number of colors.
The site requires you to scroll horizontally on a 1024 screen to see it all.