From Jared Spool’s excellent webiste, User Interface Engineering. Check out the links for detailed descriptions of each mistake:

Account Sign-in: 8 Design Mistakes to Avoid

1. Having a Sign-in In The First Place
2. Requiring Sign-in Too Soon
3. Not Stating the Benefits to Registering
4. Hiding the Sign-In Button
5. Not Making “Create New Account” or “Forgot Your Password” a Button or Link
6. Not Providing Sign-in Opportunities at Key Locations
7. Asking for Too Much Information When Registering
8. Not Telling Users How You’ll Use Their Information

8 More Design Mistakes with Account Sign-in

9. Not Telling Users the Requirements for Username and Password Up Front
10. Requiring Stricter Password Requirements Than The NSA
11. Using Challenge Questions They Won’t Remember In A Year
12. Not Returning Users to Their Desired Objective
13. Not Explaining If It’s The Username or Password They Got Wrong
14. Not Putting A Register Link When The Sign-In Is An Error
15. Not Giving the User A Non-email Solution To Recover Their Password
16. Requiring More Than One Element When Recovering Password