Despite how diverse the 00's were, now that I think about it... there were some definite standouts.
Logomania: Marc Jacobs takes over at Vuitton, and Vuitton mania sets in. Collaborators pour in, and so do the knockoffs. Dior, Vuitton, Burberry, and Gucci lead the way in branding every Oligarch's wife, WAG, chavette, and starlet from Hollywood to Tokyo and back again.
Tres Trashy: Trucker hats, booty shorts with the pockets hanging out the bottom, airbrush... all of a sudden, trailer trash was cool. Definitely not a high point in American fashion.
Sex Laws: Tom Ford at Gucci and Yves St. Laurent ushered in the 00's with a hypersexual look that reverberated all over- ironically, at this point the models from what is arguably the sexiest country on earth (Brazil) are heading out in favor of the much less sexy babydolls and Eastern European ice queens.
Different Strokes: NYC garage rock changes the face of alternative music, influencing everything from britpop (giving way to bands like Franz Ferdinand and the Kaiser Chiefs) to hip-hop (NERD and Kanye). I would personally argue that the Strokes were the most influential in this movement; since their emergence, Converse Chuck Taylor's are ubiquitous, pants legs have drastically slimmed and indie itself has really evolved.
Wonderful World: Hands down the biggest trend of the 00's was globalization. The rapid updates in the internet drastically changed how we get information and where we get it from. Now it's nothing for American schoolgirls to be inspired by Japanese ones; for London new ravers to draw inspiration from Brazillian beach bums. Bloggers have become major players in the fashion world, giving random kids from the middle of nowhere the chance to become style celebrities. The drastic evolution of the internet is definitely the single most important development of the twenty first century, for fashion and the world.