Site navigation is difficult when navigation is not permanently displayed.
Everyone likes to know where they are going and even where they are! A site with a permanently visible navigation structure meets both of these requirements, but popup or dynamic navigation systems do not. Remembering that Google and the other search engines are as likely to deliver a visitor to a subpage as they are to your home page, a clear navigation structure is essential. Think of it as they “You are here” icon in your local shopping mall’s map.
Embedded elements can clash with dynamic navigation
A typical dynamic navigation structure will result in an overlay or pop up/down display. The challenge here is that if your site is media rich (ie contains flash animation or embedded video) these embedded elements will take precedence on the page. The end result is that part of your navigation will be hidden behind these elements making site navigation difficult if not impossible.
Cross browser/platform compatibility issues
Usability issues- Dynamic Navigation is not for everyone
The Experts don’t do it
Go and visit your favorite site and then all the big internet names such as Google, Amazon, YouTube, BBC, MSN, Yahoo!, MySpace, eBay, Wikipedia, Craigslist, IMDB, etc, etc. There is a reason that you don’t see these sites using dynamic navigation and that’s because there are better ways. Any coaching or strategy expert will tell you the secret to success is to model those who are already successful. So if th336e experts don’t use dynamic navigation why would you?
You have plenty of other options