ID3v2.4 isnt that badly supported at all imho. Firefly, iTunes (and thus iPods), WinAmp, Foobar2000, Slimserver, ID3TagIT, TwonkyVision, mp3Tag, TagsRevisited… everything I have around that can play mp3s knows it. Some not perfectly, but perfect enough. I don’t know any tagger that supports encryption or compression, but those are not that needed imho 😉
ogg and flac both use VorbisComments… which are so limited. Album art support only since 1.1.3, but some people introduced a standard-violating workaround in 1.1.2 that makes album art between different handlers for those formats incompatible… VorbisComments also have a fixed position inside the file, while ID3v2.4 finally supports unsynchronization, which means they can be anywhere inside the file without harming the signal at all (good for streaming – you can even update the tags on the fly if you want, e.g. for huge one-file archives containing a full album, a some people like to store their music – with a new tag at the beginning of each track). VorbisComments may have the advantage that they’ve been planed along with the file format, but if you have a tagger that writes rubbish, you’ll not just destroy a few milliseconds like with ID3 in mpegs, but the whole file.
Slashes are not compatible to everything at all… in most apps, you’ll have a very ugly list then which makes grouping difficult (“Rock” and “Rock/Hard Rock” won’t be in the same category). Since ID3v2.3 just stops reading after the first zerobyte break, that method would still at least allow grouping by primary genre.