Key mistakes in web design

Business model - treating the web as a marketing communication brochure. Instead, adopt the new paradigm of e-services.
Information architecture - structuring the site to mirror the way the company is structured. Instead, mirror the user's tasks.
Page design - creating pages that look gorgeous and demo well. Instead, design for performance, usability, and user experience.
Content authoring - writing in linear style. Instead, break text up and write to support skimming.
Linking strategy - treating your own site as the only destination. Instead, acknowledge entry exit points across the community.

Reference:
Nielsen, Designing web usability, 2000.