Shiny Stuff Obscures Information Sites built around Java or plug-ins can't be searched or indexed by search engines. Given that searching is a large part of what makes the Web the spectacular information source that it is, there's something frustrating about designs that put concept before function. Respect the needs of vision-impaired users. Screen readers and voice browsers don't know what to do with Java or Flash. If you have a blind user browsing your Flash-only front page, you just lost a visitor.
More people are likely to pay attention when browsers and platforms become issues.
Java doesn't work well on the Macintosh. If you choose an ActiveX control, you can say goodbye to all your Mac and Unix users. Even Flash, one of the most widely available plug-ins, is troublesome when you're implementing a newer version of the software than your users have. I've been a bleeding-edge type for years. Every time the request box to download the plug-in came up in my browser, I refused to download it and went somewhere else.