>>
1- It would be nice if the links can be categorized by the user.
<<

Do you mean they should be able to create new categories and be able to move their favorites about from category to category or do you mean just group them by their category in the database?

>>
2- share favorites between users.
<<

Could you expand?

>>
3- the admin can add links to all users favorites :)
<<

Hmmm that could be done I guess.