Some main features
The ID3v2 tag is a container format, just like IFF or PNG files, allowing new frames (chunks) as evolution proceeds.
Residing in the beginning of the audio file makes it suitable for streaming.
Has an 'unsynchronization scheme' to prevent ID3v2-incompatible players to attempt to play the tag.
Maximum tag size is 256 megabytes and maximum frame size is 16 megabytes.
Byte conservative and with the capability to compress data it keeps the files small.
The tag supports Unicode.
Isn't entirely focused on musical audio, but also other types of audio.
Has several new text fields such as composer, conductor, media type, BPM, copyright message, etc. and the possibility to design your own as you see fit.
Can contain lyrics as well as music-synced lyrics (karaoke) in almost any language.
Is able to contain volume, balance, equalizer and reverb settings.
Could be linked to CD-databases such as CDDB.
Is able to contain images and just about any file you want to include.
Supports enciphered information, linked information and weblinks.
and more... (a complete list of all frames and their functions can be found here)