Dr. Nigel Puttick of North Yorkshire summed them up very succinctly:
http://photography-on-the.net/forum/showthread.php?t=113838
These are very important improvements but, IMO, the 420EX is a very viable flash unit and can often be obtained used at a very good price. I use a 420EX and a 550EX as my pair of flashes and don't really see any need to upgrade.