Browser incompatibility
While it's true the Adobe Flash plugin has been installed in very many browsers, you often need the latest version of the plugin to view content created using new versions of flash authoring software. You are also not sure that every user updates their plugins diligently. This presents the risk of your flash content not being properly viewed by all your site visitors. Woe unto you if your entire site is in flash.
Near-impossible site indexing
Site engine robots cannot execute JavaScript nor index content that is embed within flash files. For that reason, flash site designers are encouraged to provide html or text only versions of their web pages. This should however be carefully implemented to avoid penalties of web page duplication and spamming search engine robots.
Very small screen fonts
For some reason, many web sites designed entirely in flash have very small fonts that are particularly hard to read. Worse, there are no ways of resizing the page text size used on such pages. The worst part is that flash sites have no accessibility features for visitors who use text-only browsers or those who are visually handicapped.
Distracting animations and eye-candy
Many overzealous designers often go over themselves to come up with extremely busy web pages full of flashy graphics, animated banners and other multimedia flash elements. With Flash, it's very easy to yield to the temptation of creating web pages full of eye candy that fails to delight the eye but ends up distracting thge mind. Be careful, most modern browsers e.g. Opera offers users the option of instantly disabling animated graphics and other multimedia content.
Slow-loading multimedia content
It is true that most Flash content is handled as vector graphics in flash authoring applications. On the web however, these files are incorporated as SWF flash files. In view of the ActionScript that facilitates animations and interaction, the file sizes increase. Ultimately, web sites with heavy flash content take longer to load. This can easily drive your visitors away. Flash content is to be kept to a minimum.
