User friendliness:  Your links, buttons or menu, should be well organized and not spread out all over your pages. Your visitors should not be confused as to where they should go or what they should click on next and get totally lost within your site.  Your site should be easy to navigate and flow like a book.  Every area should be accessible from any page and not require clicking the back button in our browser. 

Links in working order:  Your site should be free of broken links and error messages. Visitors notice HTML errors because they will cause display problems or error messages.  Most visitors will not come back to your site to visit, if they click on a link of interest on your site which will lead them to a dead link.  You can check your links with W3C Link Checker.   Whether using text links, buttons or static menus, please make sure they are free of typos which lead to dead links or 404 error messages.  
Target links correctly:  
(External Links) Links not related to your site or hosted on another server, should be "_target blank" to open a new window. 
(Internal Links): Are links which consist on your own server/domain.

Buttons match and are organized: Buttons should match the color theme of your site.  Huge buttons are unpleasing to look at. Keep your buttons all the same size.  Buttons should not be all over your page, but should be listed all together. 

Alternative text links option: Your site should display "alternative text links" for those browsers that do not support java script or flash menus.

A back to top link should be provided: If your site requires scrolling to view your pages, "back to top link, should be provided.  Navigation should be on each page and if your pages require scrolling, navigation should be at the bottom of your pages also and not just provide a "back" button or link

Creating a site map:  Many Web sites use site maps to make it easier for site visitors to find the pages they need. For a personal web or other small web, a site map can be as simple as a list of hyperlinks to all the web's pages. You can create this kind of site map by using a table of contents.  Large business sites, should always provide a site map.