I use this tool recently a lot and there are some things that I like to share about this tool.
The thing, that the actual calculation instantly running, as soon as the user select a method, I think it is a bad practice. This is just simply wastes of time. In colored mode, there are many different settings you can choose from, in countless combinations.
Like different palettes, color comparison, color reduction mode, count unique pixels, etc. Every single time, when anything changed, it just re-runs the calculation. Because of this it is impossible to select certain combinations at once. This become annoying even if we have a tiny image, just the number of redoing the dithering makes it so slow. Unnecessary wasting of time and using the computer for unnecessary calculations.
No actual calculations should run, until the user want to do it. Add buttons to start/stop(cancel) the process.
Also related to this, to make things run even more smoothly is adding a 'preview'. Like a small window, a selected area in the image or something like that. Only show a small part of the image, which can be selected by the user. Doing the dithering just in that small window/small part of the image, saves a lot of time/calculation.
There is a thing, that I really don't like is that the software defaults to given method and color. Actually to mono and the very first method. When switchhing to color-methods, it is also defaults to given palette; Macintosh(16) sRGB. That would be nice if the program can remember the last used method. Also adding a 'settings' menu and the possibility to select what should be the default. Like different dithering method, mono or colored, and also which palette or any setting to use by default.
Why the zooming works backward? I don't think I can recall anything; any another graphic software, or even a game that using the mouse-wheel zooming by default like this tool. Even after so much time using it, still, every time I zoom the wrong way, because this reversed zoom. When rolling 'forward' that should be the zoom-in, and rolling the mouse-wheel 'backward' should be the zoom-out.
I really like those countless numbers of palettes, especially that there are palettes for old homecomputers. But I really miss the Commodore Plus/4's 128(121) color palette! Please, add it!
Also I really miss that when using reduced palettes there are no option for more than 256 colors. Like the 12bit 4096 colors on the Amiga. And the 32K (15bit) and 64K (16bit) number of colors for highcolor images.