True local interactivity. The demo was slow because it was bringing down 40 images and the network was slow. Java was designed with security in mind. Browsers can't write anything. Your applet code is checked as it is compiled.
Applets are usually small.
The speakers pointed out that the Netscape code isn't clean yet. It's not available yet on Macs, Windows 3.1, or linux. While it is clear that not everyone can use Java yet, we were encouraged to try things out and get ready for when everyone can use it.