Ah, the new hotness. Optimization, DTS audio, QT 7.4 + Leopard compatibility, MKV, subtitle and other bug fixes.
Why is this good? Perian is the secret sauce that makes QuickTime on OS X play a number of common non-QuickTime video and audio codecs and container formats. B.P. (Before Perian) was a dark period—playing video on your Mac meant using dodgy, unstable players or hacking different Intel/PPC QuickTime components from various authors/vendors. Perian also always buys the first round, gets you her phone number, listens to you bitch, cures cancer, is always up for a road trip and has the best house in South Lake.