From Jakob Nielsen's Alertbox...
Feature Richness and User Engagement
Summary:
The more engaged users are, the more features an application can
sustain. But most users have low commitment -- especially to websites, which
must focus on simplicity, rather than features.
In designing any user interface, one of your key decisions concerns the
tradeoff between features and simplicity. The more features, the more
complicated the system inevitably becomes:
* Features have to be shown to users, so screens get busier.
* Menus get bigger and/or more numerous, making it harder for users to
find the features they need.
* Features must be explained, ballooning the size of the help system
and/or the manual:
* Fatter documentation takes longer to read and makes it harder for
users to extract a good conceptual model of the system.
* More docs also make it harder for users to find the explanations
they need.
* Each extra feature offers more rope for users to hang themselves:
they're more likely to use the wrong feature, either as an error of intent
(a mistake caused when they think the wrong feature is the one they need) or
as an error of execution (that is, a slip, as when they click the wrong
button in a crowded toolbar). Conversely, Steve Jobs famously defended the
Mac's one-button mouse by pointing out that users would never click the
wrong mouse button.
* The number of feature interactions grows by the square of the number
of features: more can go wrong, and it becomes harder for users to
understand why a change in one corner of the system has an effect in another
corner.
* The more options users have to choose from, the more time it takes
their brains to prepare for action and decide what to do. Even if a fancy
feature can theoretically execute a task faster, overall system use often
slows because users spend more time on the mental operations required to
choose from among features than they save from the more efficient feature.
The answer seems clear: minimize features and chase simplicity at any cost.
This is indeed the case for most user interface design, but not for all
projects.
source: www.useit.com/alertbox/features.html