In any sort of design, it’s the little things that make the biggest difference. All those tiny details are what make up a great experience for the user. Here’s an example from pinboard. When bookmarking a site I’ve already bookmarked, it lets me know I’ve previously saved it and when:
I may be forgetful, but I’m glad to know pinboard isn’t. I don’t want to bookmark the same thing multiple times, and this is a nice solution to preventing that. Even fills in all the tags and info you entered before so you can update it if you want.
If you like these kind of touches, you should check out Little Big Details.