Flash is infamous for its poor accessibility.
Keyboard navigation does not usually work, and Flash (up until recently) did not have search engine support.
Flash applications does not work in mobile phones and other portable devices.
Flash is not there in the iPhone!!!
Flash is controlled by a single company (Adobe) and so it is not following any well defined standards for the Internet.
The beauty of Internet lies in the fact that you can always view the source code of any website you are in. This way you can use the same programming/design techniques in your website or you can find security flaws in the web application. This is not possible in Flash. In Flash, source code is closed.
The big question is, why should you use Flash “except when you are creating games or multimedia-content rich applications”?
jtyost2 says,
“I would also add that you can’t directly link to any content inside of a Flash site, thus breaking one of the major factors that makes the Internet, the Internet, links.”