The ability to independently edit the alpha channel versus the RGB channels in the DM Icon Editor is disturbingly convenient when compared to Photoshop and The GIMP.
I spent the last forty-five minutes trying to convince The GIMP to let me change the whole icon's colour to an exact RGB, irrespective of current alpha. In DMIE? Easy as pie: switch to "RGB only" and flood-fill the whole thing. In The GIMP, I have to use the "Levels" feature and explicitly set each channel's minimum and maximum threshold to the value I want, which took ten minutes to figure out all on its own.
Another problem that would be trivial in DMIE is increasing the opacity of the icon by a flat multiplier instead of just adding a value (or setting the threshold) of the alpha channel. More specifically, I'd like to increase the icon's current "thickness" to about 500%, such that an opacity 1 pixel becomes an opacity 5 pixel while an opacity 35 pixel becomes an opacity 175 pixel.
In DMIE, that's easy: Ctrl+C, then Ctrl+V four times.
In the GIMP... well, I'm still working on it. =P