The first thing when someone thinks about a website is its interactive ability. Most sites that have games and animated feedback are usually all Flash sites. A really good example of a fun, interactive site, is the
Altoids site. It has SO many fun games, and you in a way forget that you are on a site that is for breath mints. You can paint your own beard on The Unbelievable Beardo, experience the magician Sour-O that takes over your desktop, and send someone special a singing telegram.
With Flash sites, you can say what you need to say through this visual message. You can have animations that really attract the viewer in a way where they don't want to leave your site. Everything has been story boarded out and created into a movie to get the viewer to receive your message in a very effective way.
Another great thing about Flash sites, is that you don't have to worry about your site looking different on Explorer than on Firefox. It doesn't matter! It should play the same. The only thing is, that the user must have the most recent Flash player, which is easy to get and free. Someone might consider downloading the Flash player a downside, but I just think if you are one of those people who consider that as a bad thing, it just means you are lazy. It isn't hard to search "Flash Player" and find what you are looking for.
So, while an all Flash site maybe fun and entertaining, it does have downsides. We have clients, who don't really understand the web, want us to design a site where they can upload their own videos, images and content. Basically they want a site that they have complete control over, then they show us the example they have envisioned....an ALL Flash site. So here is my first problem. We CAN build you an awesome Flash site with fun extras on it, BUT once it has been created, the site you have, is done. You have no control over changing the site. There isn't a backend system where you can change out a sentence or an image. What you see is what you get.
The main reason of having a website is for a bigger population to see your company or work. And a way to do that is through SEO (Search Engine Optimization). You want someone to be able to find you on the big internet super highway easily. Flash sites hide a lot of information that can help users find your site faster.
Another downside is the loading time. Now, not all Flash sites take forever to load. But there are some sites that take long enough to lose its views. No one wants to wait more than 30 seconds for a site to load.
So, there are pros and cons to an all Flash site. But if you want a Flash site that has it all, you really have to incorporate Flash and HTML to create a hybrid site. This is extremely popular. Flash is used for the navigations of sites, main advertising, and much more. So, don't count Flash completely out, it still has that interactivity that can improve any site.