Unfortunately, many users of the World Wide Web are unaware of the rich history of the hypertext field, with many thinking it started with the release of Mosaic in 1993 or (at best) with the CERN linemode browser from 1991. Of course, hypertext goes back to Vannevar Bush in 1945 and Ted Nelson in 1965, and many of the lessons from earlier hypertext products apply to the WWW.
Sun is releasing the Java language and the HotJava browser as the latest event in browser technology. The ability to write "applets" to add dynamic behavior to the client side of the Web is powerful and will certainly lead to new and possibly dramatic applications. The closest analogy I can think of in the hypertext field is the release of HyperCard in 1987: HyperCard was also free (so it spread quickly), it was viewed as the hottest technology that year, and it gave newfound freedom to a large user community that had not previously been able to program dynamic behaviors for their Macintoshes (before the release of HyperCard, implementing GUIs on the Mac was a truly painful experience and my graduate students usually spent about two months before they could put the simplest meaningful behavior on the screen).
HyperCard had one unfortunate aspect that will probably repeat itself with HotJava: the profusion of new work included huge amounts of complete junk when people went overboard with inappropriate use of animated transition techniques, mixed weird fonts, and drew plain ugly background bitmaps. The specific attributes of bad user interface design employed by newly unleashed designers with no UI expertise will probably be different for HotJava, but rest assured that there will be some. Caution is recommended in picking up Java design ideas as long as there is no considered user interface styleguide available for appropriate use of the new technology in ways that will help users rather than hinder them.
If you don't believe that new technology can be used in ways that hinder users, then please consider the proportion of WWW pages that have used the ability to set a background texture pattern in Netscape 1.1 in ways that have no meaning other than to reduce the user's reading speed. (And given that it is about 25-30% slower to read text from computer screens than from paper, we don't exactly need page designs that reduce the readability of screen fonts even further).
My advice at this stage is to be fairly conservative in introducing new user interface elements in HotJava. A basic user interface design principle is minimalism: very often, less is more , and you can improve your user interface by removing features that do not help users. As a simple thought experiment, consider what would happen to the UI if you were to remove a certain design element: if the interface would work equally well, you should probably go ahead and kill the design element.