I started a discussion with mpt about why open source software has such poor usability and agree with his response. Much of what he says also applies to the closed-source commercial software world where good interface design is also rare. The main distinguishing characteristic seems to be polish. In comparing two programs that have similar functionality, it’s the little things that make or break it. I had hoped that Mozilla hackers would concentrate on polish bugs prior to 1.0, and to some extent that has happened. There are still too many usability problems that directly relate to polish.
Usability problems need to be considered as important as broken functionality bugs. If users cannot use the feature it matters little that it technically “works”. David Hyatt’s comment that Chimera can automatically detect and offer to block evil popups is an example of 1) understanding the target user and 2) devising elegant solutions. Disclaimer: I haven’t seen this in action. If only this user-focused mindset was more prevalent in software development.