Jakob Nielsen on Web App Usability

Interesting piece at Jakob Nielsen's useit.com. Although primarily about Flash-based Web applications, the article makes several good points about the usability of Web apps in general, to wit:

Web apps are often used infrequently (perhaps just once) and without extensive training. They must, therefore, be simple, without excessive features;
Web apps should provide an overview of features and basic task flow, so that users can quickly understand what the app does and how it benefits them;
Web users tend to ignore anything that looks like an ad. If you over-promote features of your site (with graphics, animation, etc.), users are likely to miss it.