Jakob Nielsen has a new article on his site that describes how a designer might look at conflicting requirements/guidelines and decide how to make the trade-off.
A few of highlights (emphasis mine):
In usability, this authority is provided by the basic principles for human-computer interaction — fundamental insights that have proven sound over more than twenty years of studies and research.
Simplicity says that fewer commands mean less potential for users to mess up and choose the wrong one.
Consistency says that the same action should have the same result…
It is very common to have conflicting usability guidelines. They are called ‘guidelines’ rather than ‘specifications’ for a reason: they are necessarily fuzzy because they relate to human behavior.
Interface design requires trade-offs. The challenge is in knowing how to balance the conflicting guidelines and in understanding what is most important in a given situation.
When in doubt, you can always run a user test.
What matters, though, is not the implementation, but the user experience.
The usability field is one in which empirical observation and theoretical analysis reinforce each other.
