suggestion: Please don't force the user to use UTF-8 for ID3v2.4 and UTF-16/ISO-8859-1 for ID3v2.3
The character encoding and the ID3 version should be selectable independently from each other.
What's wrong with ID3v2.3 + UTF-8
or with ID3v2.4 + ISO-8859-1
??
Example of implementation
(screenshot from program "Musicbrainz Picard"):
possible implementation in MP3Tag:
This is a very good point!
Indeed I'm just going through the same problem myself, because I just found out that my MP3 Player (Philips GoGear MP4 player SA2VBE08K) does not support v2.4 with UTF-8. 
This is the link to the FAQ about ID3 support, where it explicitly says that (in case someone is interested):
http://www.p4c.philips.com/cgi-bin/cpindex...ew_partial.html
I've been checking the ID3 v2.4 standard here:
http://id3.org/id3v2.4.0-frames
and I've seen no mention about UTF whatsoever. Weird...
So, apparently, it's not a matter of standards compliance.
Florian must have some good reason not to allow for the free choosing of UTF-8 or UTF-16, but I just don't know which it is... Might be some implementation problem...
But if there is no good reason not to allow it, then please do! 
This is the only reason why I won't be able to upgrade to v2.4.
I can't change the hardware of my MP3 player. :(altough it was one the best on the market, up to some 3 years ago.)
Florian, if you can help, I would apreciate too.
But anyway, many thanks for this wonderful piece of software.