'Single as double' click option.
There's a BeOS app. -called UniversalScroller- that allows users to set their mouseclick preferences.
Handy for those who prefer to have a left click function as a left double click, for example to reduce the risk of getting RSI.
In Windows there's an option within the OS itself (and has been there since at least Windows 98, AFAIK).
Beside the fact that UniversalScroller doesn't function in Haiku now, I think it would be a good idea to have Haiku provide (some of) UniversalScroller's 'click options' in its preferences section.