This answer is provided by Wahne Tubman, a User Experience Designer out of New York City.
The one that has the most weight in my mind is a poorly designed landing page: a page without one clear objective is a waste and limits conversions. Each page in the site has to have at least one single clear objective. The objective should be whatever task is necessary to facilitate a sale or an inquiry.
Patrick Neeman is Director of User Experience at Jobvite, a social recruiting platform and runs both the UX Drinking Game and Startup Drinking Game | More | Contact
If you're a UX Designer in San Francisco, ping me at Twitter. I want to add you to a list I have there.
