It’s been a while since I posted one of my “basic guide to the internet” articles (sorry, I’ve been a bit busy – new job and all that), but whilst you’re waiting for the next one here’s a good article by Nik about why you shouldn’t implement a web site purely in Flash.
I think it’s his third point that is the most important. Google (and other search engines) can’t index sites that are in Flash. To them (and their users) a Flash site is a black box. It might contain something useful and interesting, but who can say?
Someone once told me that the most important visitors to your site aren’t the humans, but the search engine spiders. Every once in a while, it’s worth looking at your site in a text-only browser like lynx so you can see your site how they see it.