Avoid the "Because I Can" Syndrome
When considering your audience, think about whether they have a tendency to upgrade to the latest technology or whether they use last year's browsers.
With the latest browsers, it's possible to create cool things, such as pop-up menus and floating frames, which can be moved around on the screen. Unfortunately, if your users aren't equipped with the latest in browser technology, they may not be able to navigate your site, or they may see a page that's just incoherent.
Don't use modern features just "because you can." Too many Web sites today use complex DHTML features that don't add anything to the functionality or navigability of the site, but lose significant audience because the features are not available on all browsers. Many users don't upgrade their browsers for a number of reasons:
Newer browsers use more computing resources and don't work well on older computers.
Upgrading a browser can often affect the operating system or other software running on the user's computer.
The process of upgrading can be intimidating to some less-sophisticated users.
When you do decide to use advanced browser features on your site, try to accommodate visitors who may not have support for those features. In most cases, it is possible to design a site that "degrades gracefully" for browsers that don't support a given set of features, or for users who browse with those features turned off.
Java applications can easily crash a browser, so I may turn off Java on my browser when I am just surfing so that I can avoid having to frequently restart my browser or reboot my computer.