Ken Coar complains about some of the changes in FireFox 2.0 and mentions:

My basic plaint is as usual: when changing the user interface, don’t violate the Principle of Least Astonishment and force the change on the user. Make it the default, perhaps, but always provide a preference option that lets the user keep the old UI behaviour. The user should be in charge of changing his work habits, not the software.

I couldn’t possibly disagree more. The rule is if you’re going to force the user to change their behavior – make sure it benefits them. In other words, don’t make changes that make your interface less efficient and less easy to use. Preferences are not something that make people more efficient – the preference dialog in almost every application is confusing for users and generally unfriendly, the more stuff you put there the worse it gets. You need to keep the number of preferences to a minimum – focussing only on those areas where user needs are truly different – and make your UI encourage pr enforce the most productive work habits (while balancing intuitiveness).

If you make a bad change to your UI that’s a problem, but adding a preference to revert it isn’t a solution, it’s a second problem you’ve introduced.