Here is a whole series of articles that IBM has done on the subject of usability. Usability of software is something that not a lot of people talk about. If HotSheet were much more than just a testing platform for me I’d probably spend the time to paper prototype the interface. In fact I may do it anyway because I know just how helpful paper prototypes are at smoking out serious problems in an interface with minimal work and I’ve already gotten a couple of complaints about the UI on HotSheet. BTW, some of these might sound like they are slanted only towards web usability but at least 90% of the stuff is generic to any kind of application, it isn’t just for web based ones.