In general, accessibility problems fall into several basic categories. It's no accident that most of these are about requirements; the chief accessibility problem is that sites start making demands of their users. When these demands are too inconvenient, the users go elsewhere.
Let them. In this article, I explore ways in which a designer, flush with the newfound power of authoring tools with capital letters in the middle of their names, can keep the heathens at bay, and ensure that only the chosen view his work.
Go ahead and put an "upgrade" button on your page. While you might expect this to let a lot of people in, in practice it makes no difference at all. The users who have older versions may be unable to upgrade. Maybe it's technical competence; maybe it's company policy. My mother is in the news industry; she writes for a paper. They have limited browsing capabilities. These capabilities will not be changed; they are a policy. She can't upgrade the browser. She can't download plug-ins. So go ahead, put in the "upgrade" button. Taunt her a little; power is there to be flaunted.
Insult (or ignore) blind people. A good Web designer can create a page that comes out of a text reader as "IMAGE! IMAGE! LINK! IMAGE! IMAGE! EMBED!" That's pretty amusing. Think of that laughable wanna-be customer, trying in vain to find a descriptive word anywhere on the page. Of course, this also applies to text browsers, people with low-bandwidth feeds, and others.
A menu typically consists of a half-kilobyte to a kilobyte of simple HTML, a few links, and not much else. No fun! Each menu item should be its own carefully rendered image -- this requires multiple connections from the client's browser, and may substantially increase the time it takes to finish the menu.
Frames are wonderful: They allow you to make your page totally unprintable, they make bookmarks meaningless, and they are totally unusable on small displays.