Here are the basic reasons not to use pulldown menus for navigation:
1. They are often called “mystery meat navigation” because users don’t really have any sense of how large the site is, and often times have a sense that the site is much larger than it is. While this may seem like a good thing at first (“we do more stuff, are more comprehensive” etc), users often have an immediate pre-conception that the site will be overwhelming. (Not a great brand association.)
2. It leaves the user to guide their own site experience, rather than organizing the information for them in a clear concise way, and leading them through what is important.
3. It is terrible for SEO. Crawlers have a hard time parsing through sites that use pulldown menus and often miss/eliminate key pages.
4. Many times they create display issues over different browsers and take extra development work to get absolutely right
Pulldown menus are most difficult for older users (and even not so old users) as it takes dexterity to insure that menus remain open, that the user clicks on the right link, etc.
There is also the aspect of number of clicks it takes a user to find something. This will vary menu to menu and often (as with the mystery meat concept) leaves user in a guessing game of how the site is organized and how many clicks it will take to get to where they want to go.
Pulldown menus are good for applications or intranets that the user will go to/use often, not websites. Applications tend to have a lot more information that needs to be organized and is utilitarian & users learn how to use applications. Once a user learns that “Save As…” is located in the “File” menu they can always find it there. Website users should not have to go through training on how to use a site, are of a different mindset – they tend to look for specific information only a handful of times.