Enforced immediate registration.
The long URL.
Windows that don't generate URLs.
The unsearchable web site.
Sites without Digg, del.icio.us, and Fark bookmarks.
Limiting contact to email.
Lack of feeds and email lists.
Requirement to re-type email addresses.
User names cannot contain the "@" character.
Case sensitive user names and passwords.
Friction-full commenting. "Moderated comments" is an oxymoron.
Unreadable confirmation codes.
Emails without signatures.
Supporting only Windows Internet Explorer.
Test: Can people communicate your site's URLs to others over the phone?
Extra credit: People using Verizon and can do this despite its coverage.