I think a Rolling Stone article I read summed it up pretty well, so I won't claim any credit for noticing it.
But remember how 2000 started. The best parts of the 90's were fading out, and the biggest selling groups were N' Sync and the Backstreet Boys, and the world was in the music industry's pocket. By 2010, they'd lost control. It's the decade where you if you didn't like what the stations were playing, you didn't flip to another station until you settled on whatever demographic ClearChannel had pigeonholed you into. You went out and discovered music, with or without The Machine.
All the boogeymen that the RIAA tried to convince themselves didn't exist reared their heads in a big way. The internet as a means for music, the overhead for digital recording gear becoming so cheap, you didn't need to get signed or rich to put together a great sounding record. If you had friends in music, you knew someone who could cut a good album for you on your budget. (And if you had to do it yourself, that got easier, too.)
The major labels became less of a necessary evil for alternative bands, and more of a "seriously, why would you do that?" move. Two bands I loved in the last half of the decade went "major"—Tally Hall and Motion City Soundtrack—and neither one panned out. Tally Hall even cut an album that spend years in red tape limbo after they got dropped.
So I think the 2000's is the decade where you stop getting cohesive answers to "what music defined this decade" and the question becomes more "what music defined this decade for you"?
Obligatory DLC Wishlist Reel Big Fish, Fountains of Wayne, Motion City Soundtrack, Collective Soul, BNL, recent MCR
More metal in Rock Band:
Le Desordre, C'est Moi
Resident Emo Purist/Elitist
Bands I'd love to see as DLC for Rock Band:
Bal Sagoth 
2000's was the first decade I started listening to music, so everything.
I'm going to get yelled at for this.... I think the defining music of the 2000's was... wait for it. Muse. They really started the ball rolling in 1999, and have not stopped steamrolling into the new decade. Their music has really defined a new sound for the 2000's, and their lyrics are quite deep. It's nothing new around here to know how much people love Muse around here, but it's not just a MOAR MUSE thing, they are quite influential.
We need sludgey and droney and stonery metal:
Cult of Luna
Black metal officially became a trend in music school. Split in every way possible. Symphonic and saturated or fusing with everything from indie rock to american hardcore.
Last edited by kurtdaniel; 04-23-2012 at 12:49 AM.
Well, in the widespread musical consumer world of debatable taste.
Britney Spears launched the revival of the whole girl pop thing, giving us the whole mess of Avril/Gaga/Perry/Kesha/et al.
American Idol (whilst not a specific artist) created the whole wave of quick and dirty forgettable pop singles that has basically managed to swarm over the charts.
Auto-tune has brought us tons of producers now just working as artists themselves and skipping the middlemen (Akon, Timbaland, Bruno Mars, GaGa again)
Over in band land:
Coldplay and Muse are more or less just carry overing Radiohead (who have their own 2000s work).
Likewise, Foo Fighters and RHCP are mostly just persistent 90s music.
Hard Rock seems to be set in its ways, with Nickelback and co just chugging out the same ol' same ol'.
The White Stripes have brought us a garage rock revival, and to a lesser extent, blues and country rock.
Now that we've covered all the rehashing and "revival" artists.
Gorillaz - Fused hip-hop, trip-hop, indie, and electronic, and were successful at it even. I'd give them credit for the big uprise of artists in the electronic scene over the 2000s, thats just starting to hit the mainstream (with the god-awful mutation of dubstep, granted)
dlcquickplay.com/songs/xachary_cross (Direly needs updating)