When apple introduced a one button mouse to navigate their new breakthrough user interface in 1983 there was no confusion about which button to click, no matter what task you are upto, and it was a instant hit. There have been countless debates about the decision of shipping a one button mouse, but it worked for apple for nearly 20 years.
Todays sophisticated mouse designs have a minimum of 2 buttons and a wheel which rolls and tilts, and sometimes a few buttons on the sides. And if you think you have have got the hang of its usage, wait till you open another application and many of these buttons could act differently. The simplicity of a single button is so effective that years ago apple chose the now famous one button mouse for their product.
That started our coffee table fights of how useful and intutive is the toolbar menu in our own product LyteRAD. It had started out very simple and usable, but over time all those extra features in the product have taken a toll on the toolbar. the menus and toolbar are often overlooked but they are the ones that face your customer everyday.