A client with an all-Flash website (yes, they DO exist) asked us to summarize the reasons why they should redevelop it in HTML/CSS. Here are the reasons we gave them:
Not supported on many devices (tablets and iPhones).
The website is basically empty when viewed on iPhones, iPads and other devices without Flash support. A black screen without even contact details visible.
No way to track usage.
(Without special coding) it is not possible to collect detailed traffic and usage info such as
a. what pages and content are being viewed?
b. what content is most popular?
c. where are people coming from?
d. how long do they stay on the site?
e. how did they get there? (other site? search engine? with what keywords?)
Updates and enhancements take longer and are subject to limitations.
Coding Flash is much more complicated than working with normal (HTML/CSS) web content. Adding pages, content and new features all require more time and effort.
Impossible to share specific content via social media.
Today millions of people will share links to web pages, product details, and more via sites like Facebook, Twitter and mixi. With an all-Flash site the only link you can share is to the top page. This kills all potential for social media marketing. (Special coding can help get around this, but for older sites it's a moot point.)
Flash content takes more time to load and uses more memory on the local machine.
Poor user experience.
Browser functions like the Back button fail to operate as expected with Flash websites.