Just a few small annoyances I noticed with The Unarchiver which I would like to fix if I knew how to program (couldn't figure how to build The Unarchiver project in Xcode after I was able to build libxad):

1. The character encoding chooser dialog box. For example, if The Unarchiver had trouble in decompressing 100 files in an archive for character encoding reasons, it'll ask me 100 times with a dialog box pop up. Instead of having to change selection in the pulldown menu from "Arabic" every time (apparently first in alphabetical order), like Finder, I would prefer an "Apply to all" checkbox in the dialog which I could check when I'm sure what the encoding is for all the file names. 100 selections isn't an option for me: I went and used Windows instead.
2. I got asked for the encoding on a 7-Zip archive. Isn't 7-Zip filenames always Unicode?
3. RAR archives. I noticed that for some RAR archives, it would silently fail after decompressing only some of the files. At first I didn't notice this and thought it had succeeded in decompressing everything.