The big problems with using Flash for one's website are:
Usability: Pages based on structural mark-up (HTML) can be interpreted by a variety of devices which can present the page information to users who have special needs -- there are page readers and text enlargers for people with visual disabilities, there are special input systems for people without hands, etc. Likewise, a properly marked-up page translates to a variety of window sizes, can be viewed on a pocket device and so on.
Flash cannot do any of these things. The user MUST use the interface decided upon by the Flash developer, and if they can't make use of it, too bad for them. Such users have NO access to your information.
Searchability: Flash code doesn't Google. Search spiders crawl all over your HTML and can rank your site higher depending on the <h*> tags and so on that they find. They can't find any of the information you've embedded in your Flash interface, so your site will rank lower in searches.
Maintainability: To update your Flash-based site you need a Flash developer. To update your HTML-based site you need Notepad. To change your entire look and feel and layout in HTML/CSS all you need to do is tweak ONE file with a text editor. To do the same thing in Flash requires an expensive Flash developer working in Macromedia's expensive toolset, and the chances of errors being introduced is much higher than in HTML/CSS.
Flash is more expensive, less usable and less effective than HTML/CSS for the distribution and presentation of information. It's great for specific applications, but for general web delivery, it is inappropriate.
*looks around, stops ranting wildly*
Sorry about that. Something I feel strongly about, I guess.
