I agree with >>122, the app should stay simple and not add extra features like the items in >>121.
My two requests:
Concatenate 001, 002, 003 (et cetera) files back into a single archive.
Process PAR/PAR2 files for file recovery.
Also, a request for further down the road: A dock icon, perhaps invoked when you have an archive or collection of files (e.g., a RAR set) that is over a certain size. As it stands now, it's a bit annoying to have to keep re-finding the extraction dialog buried under tons of windows (since it doesn't show in the dock or cmd+tab).
Even with these few things, The Unarchiver is amazingly sweet. Thanks so much!