Usability is a simple concept: Can people get around in your website without having to read a manual? If the answer is "yes," you've met the acid test for usability. You have to consider the usability of your Flash file and consider how easy it is for visitors to navigate your site. Is the size of your text large enough to read? Is the font legible? Does the visitor have the font installed, or are you embedding it? Are the buttons big enough, easy to find, and easy to understand as buttons? Is your site easy to navigate between each of the different sections? Sometimes you might create an "artistic" site where you specifically want the visitor to not really know how to navigate right off the bat. "Weird" navigation can be perfectly acceptable in limited situations (see http://cmart.design.ru for an excellent example), but just make sure you have considered who your intended audience is and what a member of that audience can expect to find at your site. If you were creating a shopping cart application for a business to sell its pet supplies, you wouldn't want to have three tiny + symbols at the bottom-right corner of the Stage for the main navigation! This is fine for an experimental Flash site, but not usable for a mainstream business application.
Once upon a time, Flash sites had "intros." These were often ponderous, superfluous animations that played obnoxious music and lasted forever. At one point, they were cool. And then a week went by and nobody cared any more. Seen it, we get it, you can use Flash, whoopdeedo. Where's the "skip" button?
You won't see as many Flash intros any more, thank goodness, partly because of the backlash against them. Many people hate intros and share that information with anyone who will listen. This said, many clients and individuals still love intros and create them for their websites, so they haven't completely disappeared yet. In some circumstances, intros can even be a good ideafor example, an intro that plays while a Flash game is loading into the browser cache. A best practice to follow is to "always do what the paying client wants you to do, but if you're going to make an intro, provide a skip intro button." What this skip intro phenomenon has taught us is how important usability and following good or standardized practices are when you create websites. The button immediately moves the user onto the main part of the website and stops the intro from downloading and consuming more bandwidth. Which means you'll get much less hate mail in your inbox.
What you should take away from the skip intro idea is to always remember to give your visitor control. If the visitors don't want to download something, allow them to stop it and move on. If they don't want to hear your music, give them the option to turn it off. And it is always a bad idea to take over the visitor's computer by changing the site to full screen! Dynamically load your bandwidth heavy content whenever possible, giving your visitors the control over whether they want to download the information in the first place.