Related to an applications speed, but different from it, is its interactivity. E.g. how much feedback do users get and how much do they feel in control? Wikipedias instant-publication is a positive point in this respect. But Google Wave takes interactivity to a whole new level by making the editing-process itself completely real-time. That is, as people collaborate on a document you see their cursors move through the text, and characters appear as they are typed on everybody's screen.
But besides UI-wizzardry, interactivity can also be determined by whether users are notified of updates through e-mail or other web-media. In this respect Google Buzz (a crossover between a micro-blog-post and a forum) is more interactive than Google Wave, as it seamlessly hooks into the users (Google-)mailbox. The constant notifications that Facebook sends out are another good example. Integration can increase an applications stickyness.
Allowing people to moderate, and rate things is an important aspect of interactivity as well. It gives people a sense of control. Finally, commenting, and replying to replies of others through threads, are another form of interactivity. Threaded forums make replying extremely easy. While allowing people to post a few words or a single sentence, instead of a whole post, lowers the threshold for interacting. Being able to directly respond will pull more people into using the application.