The original reason for the posts (far back in the mists of time in 2009) was that I really loathed the Rolling Stone list of top 50 albums of that time span. I felt it was so determinedly, dementedly lo-fi indie (and bad lo-fi indie at that, it's like Fugazi without the fun and the tunes) hipster nonsense that even I could do better. It wasn't like I actually listened to that much music in that decade but I was at least willing to accept there were things out there that weren't 4 boys with guitars.
I accepted that Rolling Stone were going to skew slightly to the US because that's where they're based, and mine had the opposite slant because I'm based in the UK.
So, who would I have on my list:
There's a reasonably good shout for 2000-2009's stand-out, the world stops for them, artist to have been Beyonce.
In terms of impact, to other people's sound/image, Lady Gaga has to be on there. To quote someone (sorry, I can't find who, google has failed me), "do you think Rihanna would be out there dressed like that singing about S&M without Gaga having been a huge hit?"
In the UK, Amy Winehouse changed what record companies were signing (see also Duffy, Adele, Mark Ronson) and the direction artists were going (see Plan B).
More generally, the sound of pop and pop RnB went from Dr Dre to Timberland to David Guetta.
In the UK, there was the brief but beautiful flowering of Britpop Version 2.0, so I would have had a Libertines album and one out of the Kaiser Chiefs or Franz Ferdinand. Meanwhile, Muse went from opening the bill for Feeder to arena-selling-out-colossuses (I don't even like Muse, but not having them on was when I realised the Rolling Stone list was pish). They also totally missed the rise of emo. Which is not my cup of tea at all but was still a major movement in music. It was almost as though anything less obscure than hipster indie, or louder than shoegazing was disregarded. Although at least they did have American Idiot on their list which was definitely on my top 10 list. I'm still looking forward to the 2010s-equivalent.
In the UK there was also the rise of UK hip-hop, garage and grime, or whatever you call it (this is the perfect time to drop a Wiley track -
http://www.youtube.com/watch?v=e1YKFV45M18) so The Streets, Wiley and Dizzee Rascal.
Certainly in the UK, you couldn't avoid "reality" stars and contestants, downloads started to be counted as single sales and both of the above had a huge effect on popular music. Word of mouth still mattered but it's hard to compete against the major labels and TV shows. Also, with the honourable exception of Later with Jools, there's almost no live music on TV any more which has a huge effect.