Don't break the normal functioning of the browser.
(to be written)
Don't break the Back button.
Don't break visited-link color-coding.
Don't break preview of link destination
Don't hide current location (URL).
For more information, see:
Crippling Standard Browser Features is Damn Annoying
Hiding Links is Dumb
Hiding the Destinations of links is Damn Annoying
Hiding the Current Location is Being a Pain