mIRC doesn't seem to handle files named with non ASCII characters (Japanese Kanji, or example). Every time I try to send such kind of file, the action is just cancelled without warning. Unicode support also could be extended such that scripts may use it without needing to be rewritten.

Unicode support has been discussed a zillion times, and I think the status quo is that it "might appear at some point in the not too distant future". I'm always wondering whether Japanese users will switch to Unicode or stay with the old JIS. After all, Unicode has a lot more kanji now, but still the same font problems if you combine Asian languages.