Navigation should be user-friendly and effortless to use regardless of the device being used (PC, tablet, smartphone). Links should be big enough for a finger to activate using touch screens.
Navigation titles should be clear enough so that users don't have to click on them to find out where they lead to.
The most important website content should be revealed thorough navigation as it will give a good sense of what the website has to offer to its users.
Users often ignore anything placed above graphical visuals such as banners. Navigation placed in the top area shouldn't be near any heavy visuals as most users automatically switch into 'banner blindness' mode and ignore all visuals and nearby navigation.
Grouping similar items together in navigation helps users differentiate content and see the breadth of offer. For example, items related to shopping, (shopping cart, account information, and customer service support) should be grouped together.
Navigation shouldn't contain different groups of links that lead to the same targets. This confuses users and forces them to unnecessarily make sense of it.
Users feel comfortable when they look for navigation in the horizontal area above the content or in the left column (for 3 column layouts).
A study showed that users often click active Homepage links even though they're already on it - they're trying to see if something has changed. An active Homepage link should be available from all the other website pages.
Category titles shouldn't use new terminology because it will take more time for users to find out where they lead to.
A website that allows users to shop for products or services should give access to the shopping cart on any page. This will help users quickly review what they bought without the need to checkout.
Icons can help users navigate fast but only if they are immediately recognizable. Any pause necessary to understand what the icon means is a clue that it is not intuitive enough and only clutters the navigation.