Per default KDE works in the single-click mode. The only drawback of the single-click mode in comparison to the double-click mode is that selecting items is more tricky. Instead of clicking on the items to select them, the user must either work with the rubber band or be aware that the Control- and Shift-keys allow to select items by clicking on them.
The rubber band is no equal replacement as it can select only items within a rectangular area. Beside that using the Control- and Shift-keys is a non discoverable feature it also requires that the user has to move his eyes from the display to the keyboard to hit the Control- or Shift-key (this even is valid for people that are familiar with touch typing).
Aaron came up with a nice idea some time ago, which allows selecting files with the mouse also in the single-click mode. I did not go that far yet, as I wanted a common solution that works in all view modes (even on view modes with very small icons). In Dolphin for KDE 4.1 a "select item" icon will fade-in when hovering an item:
When pressing this icon, the item will get selected and the icon changes to a "deselect item" icon:
I'm aware that this is no equal replacement to selecting files in the double-click mode, as the target area is a lot smaller. However, I still was surprised how comfortable it works especially in combination with the other view modes like details view and column view. I had some concerns that users will accidentally select an item instead of opening it, but it seems to work quite well. For sure this feature can be turned off too, as I'm aware that not everybody will like having such a fade-in icon above the items.
Anyhow I would be interested getting some feedback from people who actually worked with this feature by using the trunk version of KDE. Is the size of the icon sufficient? Are the used icons OK? Is the fade-in timeout too long or too short?